I keep a small WinXP Pro session in VMWare Fusion (along with Ubuntu, and probably tomorrow, NetBSD 5 😀), but it's strictly to run the odd windows app (pretty rarely), or to help a family member with XP.
I switched away from Windows for a reason, and OS X is just WAY ahead of both XP and Vista - and although Win 7 is looking decent (I've tried one or two of the betas so far), I don't think I'll be really impressed with Windows again unless Microsoft decides to base their OS on UNIX like Apple did.