XPpro SP2  here on the workhorse, not connected to the Internet. No updates necessary, no virusscanner, nothing. I turned it all off and uninstalled all MS apps I could (messenger, paint,...) and reentered Wordpad to write some text.

The Internet? A cheap, older laptop with everything you need: free virus scan, free firewall, Spybot S&D,...
Data are transfered via a cruzer mini stick, no network (no, thank you!).
Should there ever be a serious problem, then everything is restored to default with the acompanying CD. No serial for XPhome necessary.

Never had any problems.
O, Vista you say? No, thanks. I'm happy with what I have. And in case of real trouble, I still have Win2K to which I can return. Everything work in it, and it does not have to be activated.