Here's a great way to easily speed up your Virtual PC performance: Filter the process name from your anti virus scanner's real time monitoring.
Since I have ETRUST antivirus installed, I simply double click on the icon in the tray and go to the "Filters" tab.
I then add "virtualpc.exe" to the excluded processes list and I should be able to see better performance next time I restart it. Haven't yet tested this out myself, but it sounds promising..
(Tip passed in by Amit Marlov, passed down by Ariel Raunstein, both of which are blogless in Israel)