Linux – Windows 7 Virtual PC + Linux Ubuntu

According to this Virtual PC Guy's Blog:

It seems that people are constantly running into problems running Virtual PC on laptop systems with advanced power management. The reason for this is that Virtual PC is very demanding on hardware and has very strict timing requirements - and as each new revision of laptops comes along these systems perform more and more complex power management.

Problems start happening when these laptops tweak with things that we rely on. This can either be caused when the laptop changes the CPU frequency too often, or when the laptop actually powers down sections of the system that we are dependant on in order to conserve power. When this happens the user can see a variety of problems; ranging from sporadic pauses and bursts, virtual machines running too fast or too slow, repeated keystrokes or missing keystrokes to other timing related problems.

While I don't suggest following the suggestions in this article, the explanation might hold true.

The single biggest eater of CPU resources is probably Aero. You may test turning Aero off to lower the power demands, and seeing if the problem disappears.

In case it does, you may set Aero to turn off automatically when and only when VPC is run, by right-clicking its executable, choosing Properties / Compatibility and checking “Disable desktop composition”.

not really the best solution, but if nothing else works - Windows Virtual PC now has a patch enabling to work on non VT-X machines, so you could try that, although it would be slower.