How Do I Fix My Computer If It Keeps Rebooting?

Eliminate Hardware Issues
Hardware failure can cause a never-ending reboot cycle; eliminate the most common causes first. Replace memory chips one at a time to rule out a bad stick of RAM. If the memory checks out, boot from the Windows CD and access the Recovery Console. Run chkdsk /R at the command prompt to check for problems with the hard drive. Remove any optional cards that were added in to the computer (such as a sound or network card) to eliminate them as the cause.
Eiminate Software Issues
Press the F8 key while the computer is starting up to access the advanced start-up options. Select 'Last Known Good Configuration,' and press Enter. If that does not work press F8 again, and select 'Safe Mode' this time. If you are able to access Windows in Safe Mode, disable the option to automatically restart Windows after system errors by right-clicking on the My Computer icon and selecting 'Properties.' On the Advanced tab, click the 'Settings' button under 'Startup and Recovery' and clear the check box labeled 'Automatically restart.' This will allow you to restart the computer without getting stuck in the reboot cycle. Ensure that your antivirus and antispyware software is up-to-date, and perform complete scans for any infections. Remove any newly installed software that may be incompatible with Windows, and update drivers for your hardware.
Eliminate Power Issues
Replace the power cord to the computer, replace the power strip it's plugged in to, and try a different outlet. If nothing else helps, try a new power supply. Be sure to purchase one with enough power for your computer: use a high-performance power supply for a gaming computer or a lower-performance power supply for a regular workstation.