Just keep in mind that Boot Camp is not a 'program that runs Windows'. So, it's not about what's compatible with Boot Camp as much as what will run well on your hardware.
You didn't mention exactly which Mac you have, so it's hard to make a recommendation. In general, XP is a solid choice, although it is a bit long in the tooth at this point and eventually MS will drop support (most importantly, security updates) and 7 tends to be a bit more resilient with its UAC scheme.
Additionally, 7 will be able to offer you DX10+ capability if your hardware supports it. XP is limited to DX9. That's something to consider if you're playing newer games.