I have formatted my system six times in this week. My system is continually restarting. Re-installation of Windows dose not work. Is there any way to make my system well and to stop this problem?
Do you hear any beeps or noises before it re-starts? Or any errors? I would start testing for the fan. That issue is very common with computers when they overheat. Open the case, and place a fan directly towards it while you work. If it stops rebooting, it's the fan.
Sounds like you are getting the Blue Screen of Death and windows is automatically restarting. Follow the procedure in Saikat's link to ensure it doesn't automatically reboot.
As the above people mentioned it is probably a hardware issue.
By the way, it doesn't restart every 2 hours does it? If it does, use a different operating system than Windows 7 RC :)
Sounds to me like it could be a hardware issue too. Maybe the power supply or hard drive. I would explore other options like different operating systems before replacing anything though.
If re-installing Windows doesn't help I don't think those steps will help; it sounds like a hardware issue to me. See if Saikat's advice helps, but if not I'd recommend booting up a Linux CD and seeing if the rebooting happens on that platform as well. I recommend Ubuntu:
If this restarts in the same way we'll know for sure that this is a hardware issue.
I had this problem in Windows XP once. The solution is actually simple. Check out the advice given in this link.