Does this computer have a fan on the main chip? Any chance that its stopped working and the main chip is getting too hot, and auto shutting down? Have you checked the fans inside the power supply,...

When you reboot the computer, what do you see? Are you using Windows Explorer or My Computer?

One sure way to get around this is to partition your hard drive into two sections. You can do this either before installing anything on it in the beginning, or using an aftermarket program...