Not meaning to sound like a techno-snob, but have you considered Linux? Most malware is designed to run under Windows. It won’t run on the wrong operating system. Give Ubuntu a whirl and see what you think. It comes on a “Live-CD” meaning that you can run it temporarily just by booting the computer off of the CD-ROM.
If you don’t mind spending money on new hardware, which you’d have to do anyway to upgrade to Vista, go with a Mac.
Oh, I forgot to mention that PCLinuxOS is a live CD as well. Try before installation. There’s a whole list of Linux Live CD distributions here:
http://www.frozentech.com/content/livecd.php
It’s worth a look!