A legit copy of Vista should work fine. I have had Vista runnign under bootcamp before.
On the virus issue, if Bootcamp is running your machine IS a PC with all the attendant dangers, though Vitsa's UAC and other bots have added some greater protection. I woudl say your PC partition is as in danger as any other PC, what I'm not sure of is the ability of the Bootcamp partition to damage the mac partition. I know I used to be able to drag and drop from one partition to the other so there is some danger of damage I guess, maybe someone else knows more.
Best you can do is protect the Win partition. Get hold of some antivirus/spyware defense. Windows firewall under Vista is OK, then pick up the AVG free version, or avast, both free antivirus packages. Comodo firewall pro is also free and good, though it wound me up with the endless security messages so I got rid of it. I think I'm still using Adaware against other dangers, I forget and am not near my PC right now.
