Get a Mac, run VMWare Fusion to create a windows virtual machine, and use the snapshot feature often.
That way, when a virus gets to windows (and it will), you just restore to the virtual snapsot and keep on truckin’
I like my mac without the virtual windows:)
I doubt that will run on my 400MHz Power PC G3...
If you switch to Linux, you could accomplish the same thing and still keep your current hardware.
I run Win7 (RC candidate) in VirtualBox on my Linux laptop so I can sync up the iPhone with iTunes. That's about the only reason I run Windows anymore.