There is a hotfix available to fix a bug where Microsoft Virtual PC 2004 (with or without SP1) doesn’t deal very well with Intel Speedstep technology in the Intel Centrino 915 chipset (e.g. in a Dell Latitude D610 and many others) – basically the whizzy power saving features of the laptop can cause strange CPU and clock behaviour for any guest operating systems.
There is a kb article here: http://support.microsoft.com/?kbid=899525 which describes the issue and the steps for resolution.
Judging by the dates on the patch file, it seems the update has been around for a number of months, and has seemingly passed me by!