为什么我的计算机会自动关机和重启？

请教各位高手：

I have a Compaq Presario P4, 2.5GHz desktop --2 years old. Recent few days it started to show something odd. From time to time, the computer shuts down itself and then automatically trys to reboot. Sometimes it lasts a few hours, but more often it keeps shuting down and restarting. if I leave it unattended, it keeps on and off without stop.

I am really worried about it and start to copy my useful files to a external hard drive.

Anyone knows why and please give me some hint? Appreciated.