In an article titled Tuning Virtual PC Performance on WindowsNetworking.com, I gave a number of tips for improving the performance of Virtual PC. Here's yet another tip for you:
If you have antivirus (AV) software running on your machine, configure this software so it does *not* scan the files used by Virtual PC. For example, it should not scan your .vhd or .vmc files i.e. the contents of your My Virtual Machines folder or the separate drive where you store your .vhd files.