Vista

Yes I've been lazy.

http://www.eweek.com/article2/0,1895,2062318,00.asp

Microsoft really doesn't want you to know this, but many of your existing applications won't work with Vista. In fact, some brand new products won't work with Vista.

At the top of that list of "It doesn't work," we find, believe it or not, Microsoft new music player, Zune. Yes, that's right; Microsoft's iPod killer doesn't work with Vista.
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Then, consider if you will, Windows Live Messenger 8.0 and the beta 8.1. Both IM clients blow up after a few hours of use on Vista.

Oh, and the new IM client from AOL, AIM 6? I can now attest from personal experience that it goes "boom" too on Vista.
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That said, I do wonder a bit why many major anti-virus programs, such as AVG Free, Symantec AntiVirus Corporate Edition for Vista, and Kaspersky Anti-Virus & Kaspersky Internet Security 6.0 are all reported to crash and burn.
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You should also know that some of Microsoft's own programs won't run with Vista. For example, Microsoft's own Virtual PC 2004 won't work.

Neither will Visual Studio .NET 2002, Visual Studio .NET 2003, and Microsoft SQL Server 2000. Visual Studio 2005 SP1 will work... sort of.

So, if any of you developers out there are planning on creating.NET 1.1 applications in Vista, forget about it!

And that should be quite enough.