Well actually I dual boot vista(Evaluation) and XP, just for checking it out...
I dont install programs twice, ie on each OS...
In XP, one file that I ran had a virus, and because of it, literally every system file in XP had a virus. I ran this McAfee, and like atleast 150-200 files infected in the C:\WINDOWS folder alone.
Now in Vista, the same program was run, and when I run McAfee, only 1 file infected, that too it's the virus file which I originally took. No other system file, game, office file, no nothing was infected...oh and BTW that virus was called W32.Sality/x
Isn't that cool?