I'll have to second that, and add a little more insight as well... VirtualPC uses one, big file to store the Windows OS as well as any other programs you load into that virtual OS... so any virii would be contained within that file as well. The only way you could get the virus even ONTO the OS X portion of your disk would be to copy it there. And, as davidbrit2 said, it wouldn't run anyway.
The only thing that a Windows virus can harm in Virtual PC is the Windows OS you have loaded in Virtual PC. And if you have more than one Windows OS loaded in Virtual PC, the virus is going to be contained to the Windows installation that was affected -- it's not gonna "jump" to your other Windows installations.