I just experienced the same. I had Parallels up and running WinXP. The XP installation had taken me quite some time to create (CVS setup, Editor and other software). The lag on the mouse movement annoyed me so I thought I'd install Parallel Tools since they suposedly offer better speed. Just seconds after the Parallel Tools installer had started, my MacBook Pro restarted.
When it had restarted, I opened Parallels and tried running the VM. I got a message saying that the VM was being used by another instance of Parallels and that it could cause problems if I tried to run the VM anyway. I tried another reboot to no vain. I then tried running the VM despite the warning.
Now, the VM refuses to start XP. It shows the intial DOS-screen and displays the loader bar (the one that is made up of ASCII characters). As soon as the loader bar is filling up, the VM just quits wth no warnings and no dialogues.