When you use Parallels to run Windows inside of OS X, some of the hardware is virtualized. When you use Boot Camp to run Windows natively, all of the hardware is what's actually on your machine. So what you have to do is use two different hardware profiles inside of Windows - one for your "virtualized" hardware set (running under OS X) and one for your "native" hardware set. Just be sure you name them in a way that makes it easy to distinguish at Windows boot time.
Thanks for the clarification. Indeed, the setup you describe is precisely how the Parallels setup is supposed to work. My experience (and that of others on the Parallels forums) is that is does not. I'd be interested to hear if anyone using a version of XP that requires activation is not having these problems.