I've had a lot of trouble with this on my Dell laptop. I thought I'd share my solution to the problems I was experiencing (slow down then sudden speed up of keyboard entry and screen writes, intermittant response etc). Use a bit of software called SpeedswitchXP from http://www.diefer.de/speedswitchxp/ to stop the intel processor from switching power modes. I think these are sometimes called c-states. It works a dream. Virtual PC / Virtual Server become completely usable again. I tend to run it with the settings for maximum cpu speed. Doing this still gives me about 80 minutes on the battery - more than enough time for the trip in or out of work.