Ted Neward posts that he's having trouble with Virtual PC 2004 SP1.
My guess is Ted is running it on his IBM ThinkPad T42p. My suggestion? Turn off the SpeedStep feature of the processor in the BIOS. (You can also just try setting your power management settings to Always On). You can verify that your processor is running full speed by downloading Intel's Processor Identification Utility here [0].
There seems to be some issues with laptops and SpeedStep and the SP version of Virtual PC. I also seen reports of problems on Virtual Server with laptops.
End of line.
[0] http://support.intel.com/support/processors/tools/piu/