Microsoft Windows XP service packs.
I've used Microsoft Windows Vista Ultimate 64-bit for over a month now and I'm quite comfortable with it. Aside from the GUI, there's little change from Windows XP, but it supports DirectX 10. Just one Blue Screen so far.
Microsoft Windows XP Service Pack 3 is due out tomorrow, 29th April 2008, but the file is apparently already on the Windows Update server. If you're wary of authenticity from third-party sites, here's the link straight from Microsoft itself.