Microsoft's VPC
Q&A page should give you a few pointers about how it works.
Basically, as it says on the page;
Virtual PC is an emulation program that creates a separate Windows environment on the Mac desktop. Through some sophisticated software trickery, Virtual PC creates a "virtual machine" that runs right on your Mac. This virtual machine is much like a real computer — you can install an operating system on it, such as Windows XP, and you can install programs, such as Microsoft Access or Visio — all it lacks is actual hardware.
The virtual machine appears in its own window on your Mac and you can switch easily between programs on your virtual machine and programs on your Mac.
I'll freely admit to not using any of the more recent versions of VPC so I'm not saying this with first hand experience, but please be aware that as it's an emulation of the OS (your G5 will still be running OS X) you're not exactly going to get speedy performance. For one thing, if you were looking to run games on there you'll probably end up more than a little disappointed.