Dont quote me on this, but since the Virtual PC uses a closed .vhd file on your computer, it should not escape the boundaries of that file.
I would think you would be pretty safe. The virus would have to be smart to see that the virual hard disk is actually a file of a filesystem contained within another file system. It cant do any damage to your other files accessable by your native OS
Just as always, exercise some caution and think the worst first.