Scott Hanselman is one of the great folks in the community who are a part of Microsoft's Regional Director program, and an excellent collector of tips on a variety of subjects. I've referred to his speaker tips, which I posted about here, on many occasions.
Now I noticed Scott has a very nice collection of tips for improving your experience with Virtual PC, including information on enabling hardware assisted virtualization:
I just enabled the virtualization option in my BIOS (Dell Latitude D820 with the Core 2 Duo), and Virtual PC picked it up automatically. I haven't played with it enough to be sure, but it does seem a little snappier, so I'm glad I found out about this.