I'm dual-booting right now (on my brand-spanking new Dell with 19" Flatpanel monitor
). As I'm a web developer, I use debian linux (120GB partition), but I have 39GB for Windows. I mostly use that for games, and, if ever the need arises, I'll use it for complicated word processing.
Anyway, all of you who are backing windows have valid points, but forget the usual hole - Internet Explorer. My XP is fine, runs fine, plays my games fine, runs my Office 2K3 just fine. But I don't trust Explorer with my browsing. And considering that using the internet is a primary use of a computer these days, I'm not surprised at the number of people who bitch at MS after having to yet again remove some spyware/adware...
I started up IE once. It went to the xtramsn site (New Zealand homepage for Windows XP). I went straight to mozilla.org/products/firefox, then to mozilla.org/products/thunderbird (though I won't be answering my e-mail in XP for a loooong time). Then I shut IE down, and ran a batch script to disable it.
If I ever connect to the internet in XP, I'll do it through Firefox, with the AVG virus checker running. That way I think that XP will remain my first choice for games and word processing.
However, Linux remains safest for internet browsing, and generally better for development of any sort (unless you're a VB/ASP nut...)