Why anyone would run that shit when there is VMware, which is 358673094573x better, I have no idea.
VMware only virtualizes the x86 CPU. Virtual PC emulates an x86 for the Mac users.
VMWare is better on x86 because it doesn't translate. VPC on PCs still "emulates", therefore losing a lot of the speed that VMWare gets.
All VMware does is send the instructions verbatim from the virtual CPU to the real one, with some overhead from having to keep the OSs separated.