Starting VM makes Mountain Lion Reboot

I am running the latest downloaded version of Parallels 7 on Mountain Lion OS X 10.8.2. I have tried creating a virtual machine with my original CD copy of Windows 7 Pro, with a downloaded preview copy of Windows 8 and using bootcamp with an existing Windows 7 harddisk partition. The virtual machine seems to install properly in each case, but when I run any of these VM's, the MAC just reboots. I don't even know where to start trouble-shooting since I'm not getting any error message. Any thoughts on what could be going wrong?