Today I booted a virtual machine that was provided for me by a client so that I could prep a training course, but I could not log into the virtual machine because it blue-screened with a driver called intelppm.sys failing (and supposedly processr.sys can cause the same problem, but I have not experienced that one yet). Note that the virtual machine might just reboot rather than blue-screen because that is what it is configured to do.
The blue-screen is due to the virtual machine running on an AMD K8 or Centrino platform and the above device driver attempting a unsupported command that was supported on the platform that the virtual machine was originally created under. You can disable these drivers without issue in the virtual machine - just you cannot login to disable them!
So you need to use Safe Mode. Press the F8 key repeatedly as the virtual machine boots and then choose Safe Mode from the on-screen menu. Once you have logged in run these two commands from a command prompt (note that where the spaces are is important):
sc config processor start= disabled
sc config intelppm start= disabled
You should get a SUCCESS message returned. Once you do you can shutdown the virtual machine (and commit changes if you are have undo disks enabled) and then start the machine up normally.
This problem has been fixed under Virtual Server 2005 R2, but is still an issue with Virtual PC 2004 and Virtual Server 2005.