windows keeps restarting

Which OS?

If you are using WinXP, you probably have one of the following virus:
MIMAIL
BLASTER
WELCHIA

Virus removal tools can be found here:
http://securityresponse.symantec.com/avcenter/tools.list.html

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Win2000? You may be experiencing software and or hardware issues. Try booting to Safe Mode and going from there. chkdsk /r ....

It could also be a failing power supply or main board failure. Also consider a faultly ground connection. Try plugging the computer into a different socket in a different part of the house/office. Make sure you are on a different circuit.

If you are using a surge protector and it is old try replacing it.

If this is a 98 OS try booting with the start-up disc. Be sure to lock the 'write protect' tab in case you do have a virus.

Depending on which OS you're using you can press the F8 key as soon as the computer starts to boot. This should give you a "boot menu". Choose the step by step startup. You will have to answer yes or no to all of the device drivers as they load. Choose yes. You will now be able to see at what point during the boot process the computer is restarting. This will at least give you a good starting point for further trouble shooting.
This same problem happened to me a while back and it turned out to be a bad hard drive.