How far does it reach before it restarts itself? Upto Windows Splash Screen? or before that? What is the operating System installed on your system? i.e. windows xp or vista or etc..etc... In the first part, it seems a hardware issue, but when I read "Once it's running, it's good...." it seems application issue. I can't make sure right now. We need to diagnose and do some analyses.
The first thing you can try is "
Safe Mode", boot into
Safe Mode several times and verify whether the problem persists or not. If the problem persists, you need not to do anything, but if there's no problem booting into Safe Mode, then uninstall installed programs which you could reinstall later, clean the system's registry files, delete temp files (use this program to do those cleanings
[Click Here to Download]). Uninstall and reinstall the Antivirus Program or any other security programs like anti-spyware [never install two or more antiviruses on a system]. Now try again and check whether this helps or not.
Also disable all the Start Up programs except Microsoft's programs, restart your computer and try again. To do this, press "Windows Key + R" [Windows key = the key on the keyboard with Microsoft Logo], type in "
msconfig" [without quotation marks], press enter, click "
StartUp" tab and then "Disable All", click "Ok" and again - restart your computer several times. If this doesn't help. Then the problem must be with the Windows or the Hardware(s).
Let me know the result so that we can proceed........ All the best.