Man, I'm tired... let me try and think straight...
Okay, its not necessarily illegal to put Mac on a PC... however, it is illegal to "tamper" with the programming, modifying it so it can go on if for some reason it needs to be modified.
Assuming that the poster isn't modifying the program to install it on a PC, yes, its possible to to so.
You can certainly install Vista, or XP, on an Intel based Mac. G1, 2, 3, 4, whatever other Mac processors there are, will not support Windows.
Usually, because of Windows' great driver support, it will work fine. You might run across issues with special customized Mac hardware though... but, it really shouldent be a problem.
As PCWizard mentioned, there are quite a few pieces of software that are capable of doing this...
I would recommend Bootcamp. Bootcamp is basically duel-boot software for the Macintosh. Its free, and developed by Apple.