Aha.  I decided to revisit this tool to test XP SP2.  I did another search through Help to see if I could correct the performance issues I ran into last time, and this time I uncovered the following:

To turn on the pausing of background virtual machines

1. Open Virtual PC Console.
   
2. On the File menu, click Options.
   
3. Click Performance.
   
4. In the CPU Time list, click Pause virtual machines in inactive windows.

 Note

• To open Virtual PC Console, on the host operating system, click Start, point to All Programs or Programs, and then click Microsoft Virtual PC.

I'm going to give it a try....