Virtual PC is not the same thing as booting windows XP on a mac.
Virtual PC is a x86 emulator that runs on Macs using the G4 or G5 cpu. VPC runs as an application in OS X much like the way Classic runs. With VPC you can install Windows XP (or linux, or any other x86 OS) and the result is that you have an application that you can launch that runs Windows XP inside the application. The main drawback is that Windows running inside VPC runs fairly slowly. It is usable, but not fast.
The current version of VPC will not run on an Intel based Mac.
Running Windows XP on an Intel based Mac (like the MacBook Pro, Core Duo iMac and Core Duo Mac Mini) has nothing to do with VPC. Since these new Macs use an intel CPU then it is possible to install Windows XP directly on the hardware --- replacing OS X, or alongside OS X so that you have the choice of booting into OS X *or* booting into Windows XP. At this stage it has only been possible to run Windows XP on an intel Mac for a few weeks. The install process for Windows XP on a Mac is tricky and at this stage there are still a lot of problems with it. Over time it will become more stable but its not ready for the average user yet.