Unless your insides' of your computer is CAKED with dust, and no fans are spinning, then I don't think it's a heating problem. LIke bewildered said, sometimes that's what it is. However, I, myself, believe it's a windows system error, something through either your VM, or windows dll files, are causing restarts, IF your trying to run something particular. If it's just out of no where, and your idle, MOst likely it's hardware, and check what bewildered said. Window's own addressing system, and the way windows saves files, might have become corupted, during something that you have done previously that didnt' go through, such as a freeze, while it's saving something, and you stopped it from saving, or rebooted your system when you wasn't aware that something was being written to the hard drive. Usually a reinstallation of windows could be your best bet. IF your HD's contain bad sectors, that can cause freezes, and restarts as well. Running a deep scandisk can usually isolate those sectors for non-use. IF you tried to overclock, or whatever, it's probably your memory being bad. IF you don't have a sufficent amount of memory, and you run your system HARD for even a minute, and run out of resources, your system can restart. There's numerous things that it COULD be, but troubleshooting, and eliminating the non-possibilities would be your best bet. Start with checking for overheating (by softwares and then open your case). Take out any extra cards you have, like modem's, sound cards, blah blah blah, and if you have 2 sticks of memory in there, take one out, and run 1 hard drive (your main one), and play around it, and see if it restarts on you. If it does, change memory sticks (if you had 2), if you didnt' have 2, run safemode, and see if it'll restart on you then. If it doesn't, than it's software issue itself (even if it sometimes does, than it still can be software issue).
