I'm at Microsoft's MIX conference in Las Vegas, where the big news has been the unveiling of the Windows Phone 7 development platform, and the platform preview of Internet Explorer 9 with extensive HTML 5 support.

There is more to say about both topics; but one thing I want to highlight is that IE9 will not work on Windows XP, the venerable release that will not die.

It is not only that XP remains in use on existing PCs; there are also new machines on sale with this old version of Windows. I've just purchased a netbook, and when making my selection I noticed that many of them still come with XP, usually the Home edition. Some companies still specify XP when buying new PCs, to avoid the compatibility hassles that come with a move to Vista or Windows 7. There's also doubt over the benefits of upgrading. A friend said to me recently, "I really like XP, it does everything I need. What is the point of moving?"

Here's what IE General Manager Dean Hachamovitch told me:

Building a modern browser requires a modern operating system. There are facilities in Windows Vista and Windows 7 around security, for example the integrity level work that gave us protected mode, there are performance improvements that enable a variety of things in the browser, there is graphics infrastructure to take advantage of the GPU, that doesn’t exist in previous operating systems.

A measure of scepticism about such comments is reasonable. Microsoft wants users to buy its latest stuff; there's no surprise there.

Nevertheless, Hachamovitch is right. Windows 7 is more secure and more powerful. Personally I find it easier to use as well, partly thanks to Microsoft's design work on the user interface, and partly because desktop composition in Windows Aero enables richer preview of minimised applications and other good things.

Microsoft is also making a statement with IE9, to the effect that XP users can no longer expect to be included in major product releases. Office 2010 does support Windows XP; but you can bet that the next one will not.

The long life of XP is a side-effect of one specific thing, which is the failure (relatively speaking) of Windows Vista. I used Vista from its first release and regard it as better than its reputation suggests; but nevertheless, it was greedy for hardware resources, prone to annoying slow-downs, and less polished overall that it should have been.

There is also a real issue with application compatibility, introduced with Windows Vista, mainly thanks to User Account Control. This feature protects access to system locations such as the Windows and Progam Files folders, and parts of the Windows Registry, causing problems for some applications that expect full access.

These factors extended the life of Windows XP, resulting in the situation we have today: an operating system coming up to nine years old still in widespread use.

Very often the continuing use of XP is not something we can control. Rather, it is something we have to live with. Nevertheless, it is becoming a liability. Windows 7 improves on XP in every way that I can think of; and we even have XP Mode and Med-V to assist with migration, by running obstinate apps in virtual instances of XP.

Windows XP is something we have to live with, but no longer something to recommend.

Postscript: arriving at the gate for my flight from Las Vegas, I could not resist snapping what seemed the perfect illustration: Windows XP with a sad little error message.

Microsoft has completed Windows 7 and released it for manufacturing with much fanfare. It is a key step in terms of Microsoft's battle with Apple for hearts, minds, and above all sales; but does it matter to the average corporate developer?

There are a couple of ways of looking at the question. Windows 7 has new features, and although it is unlikely that you would want to require Windows 7 for your application at this early stage, you might want the kudos of adding a few enhancements that appear when running on the new operating system.

The other angle is simply this: if some early adopter insists on running your app on Windows 7, against corporate policy, will it still work?

Windows 7 is an unequivocal improvement on Vista and makes XP look distinctly dated and insecure, so it's likely that adoption will be relatively rapid, especially in the consumer market.

The official line is that application compatibility should be good, and my experience bears that out. In particular, if an application runs OK on Vista it will almost certainly be fine on Windows 7 as well. An XP application is more likely to break on Vista, than a Vista application on Windows 7. User Account Control (UAC) is the biggest source of compatibility issues, particularly for applications that assumed the user to be running with local administrator rights. Test your applications, of course; but unless you are writing tricksy things like anti-virus utilities they will most likely work as well on Windows 7 as they did on Vista.

What about enhancements? Some Windows 7 features are worth supporting, because they improve usability. The most obvious example is the taskbar, the heart of the Windows 7 user interface. Taskbar jump lists, which are right-click menus for taskbar icons, can be customised to support tasks specific to your application; and taskbar preview windows can include controls enabling a miniature user interface for the most common actions. This means users can do some of what they need without opening the main application window at all, which can be a great time-saver.

So how hard is it to add Windows 7 features to your application? An interesting question. Despite the popularity of .NET Languages like C# and Visual Basic for business applications, Windows is mainly written in C++, and when it comes to supporting new features that shows. If you are a C++ developer, you can download the latest SDK and get going immediately.

.NET developers have a more difficult set of choices. You can wait for Windows 7 features to be added to the .NET Framework (it could be a while), or roll your own interop code, or use one of Microsoft's .NET libraries, which are as yet incomplete. The last route is the most tempting, though Microsoft has gone out of its way to confuse matters by producing both sample libraries targetting specific areas, like the Windows 7 Taskbar and Libraries .NET Interop Sample Library, as well as an all-embracing Windows API Code Pack which is I think the preferred solution but which is in some respects even less complete.

Still, it should not be too hard to add some Windows 7 bling to your app using one of the .NET offerings. Then again, hasn't the boss started using a Mac at home? Is it better to invest your time in polishing the experience on Windows, or looking into how you might port to web-based and/or cross-platform applications? Like everything else, it depends; but if you work in .NET it is well worth keeping an eye on Silverlight as well as the latest developments with Windows 7.

Other links:

This official blog post is an overview of the main API changes in Windows 7.

The Windows SDK blog has detailed info and gotchas for bleeding-edge Windows developers.

There is a kind of Windows 7 fever sweeping the Web right now. Some observers are getting carried away:

As a person who performs almost every computing task on a Mac and tells anyone who will listen that at this point, the average consumer should be using a Mac instead of a Windows machine because of security and usability, I'm starting to prep myself for the single moment that I thought would never come: I'll be using a Windows 7 machine as my main computer and telling anyone who will listen that, believe it or not, using the latest Microsoft operating system really is worth it. [Don Reisinger]

I've spent a lot of time in Windows 7 these last few weeks, and in some ways I see what he means. It has a smoothness and elegance that is lacking in Vista, though it is there to some extent in Server 2008. But let's be clear: the popular view that Vista is rubbish and Windows 7 is great does not stand up to close analysis, for one simple reason. It is just not sufficiently different. Microsoft actually makes a positive of this characteristic, explaining how preserving the core architecture of Vista ensures good application and driver compatibility and therefore a smooth upgrade.

What about Windows 7 versus the Mac? I agree that Windows 7 is superficially more Mac-like; and more important, that the factors that made Vista such an unpleasant experience for some (not all) early adopters will not apply. I'm thinking of things like laptops that were underspecified, or had buggy drivers, or barely adequate graphics hardware, or that were weighed down with so much foistware that Windows would hardly run. However, other factors that make users prefer Macs will not have changed. There is the control Apple exerts over both hardware and software; its design excellence; the fact that Apple is less tolerant of legacy software, whereas Microsoft has Windows jump through hoops to keep it running,adding complexity and inconsistency; and there is the absence on the Mac of the culture of chaos that afflicts Windows. I'll leave aside religious arguments about Unix vs Windows, though it is a matter of record that OS X has to date proven far more secure, for whatever reason.

Let's ask some awkward questions. Will a substantial number of Windows 7 machines fall victim to viruses, worms, trojans and botnets? Almost certainly. Will Windows 7 from time to time flummox users with obscure errors like "the system could not find the file specified"? Almost certainly. Will some Windows 7 machines be built down to a price and be sold with obvious design flaws and insufficent attention to quality? Almost certainly. Will some vendors wreck the sweet install experience Microsoft has created by imposing their own clunky utilities and third-party trialware on top? Almost certainly. Will the Windows 7 event log become populated with perplexing entries like "The Workstation service terminated with the following error: The redirector is in use and cannot be unloaded." (plucked from my Windows 7 beta 1 laptop)? Almost certainly.

Let me add that I realise how smooth and reliable a well-managed Windows machine can be. Further, while I have had some frustrations with Vista it has always been stable for me and I have never wanted to go back to XP. Vista, then, is better than its reputation; Windows 7 is better than Vista but will have trouble living up to the proclamations made by its more enthusiastic admirers.

I'm at Microsoft's Professional Developer's Conference (PDC) in Los Angeles, where we've heard a ton of stuff about Microsoft's forthcoming technology. A lot of the press has focused on Windows 7, and that's understandable since Windows is what many of us stare all day. I've been running Windows 7 myself since Sunday, in an pre-beta build, and I'm both impressed and unimpressed.

The good bit: Windows 7 is better than Vista in every way I can think of. Even in the pre-beta, it is fast and stable. Even better, Microsoft has worked on making Windows "quieter" - reducing the number of distracting dialogs and notifications, and giving users more control over them.

Too much "toast" popping up in the system tray? Just choose "Customize", and you can turn off notifications from applets that are annoying. Too many prompts from Vista's User Account Control, the thing that flashes the screen and asks, "Did you really want to do that"? Now there's a simple slider that lets you minimize the prompts. Provided that you avoid the lowest level, security is not much compromised.

There are other user interface changes, but the nagging question is whether Windows 7 really merits a full new version number. In fact, Microsoft says there are no core architectural changes, which is great for driver and application compatibility, but reinforces the impression that this is just Vista done right.

The biggest innovation (if you have never seen an iPhone) is the multi-touch control, which lets you use your fingers instead of the mouse. You can scroll windows with a flick of the wrist, and pinch the screen to zoom or rotate what you see. Impressive; but whereas this works well on the iPhone which is designed from scratch with this in mind, there are a couple of problems applying it to Windows. First, most of use don't have touch screens, and while that might change, it's also possible that the technology will go the same way as the current Tablet PC, into a small niche. Second, how many application developers will make the effort to support touch properly? Watch this space; but I guess it is possible that mouse and keyboard will remain by far the most common way to control Windows.

The more interesting themes at PDC are outside Windows itself. There's cloud computing, there's Visual Studio 2010, there's news on the future of C#, which as its architect Anders Hejlsberg pointed out, is now a decade old, and plenty more. I'll post separately on some of these topics.

Nevertheless, Windows 7 will be a welcome upgrade when it comes. Which is when? Microsoft won't tell, but I'm guessing we may have it in our hands by this time next year, probably earlier. OEM vendors will want it for the Autumn. To hit that date, Microsoft will need to be complete the OS by the summer. Given the lack of major changes under the hood, that strikes me as plausible.