windows keeps rebooting

i boot my computer up into windows xp pro and it asks me for my user name and password i hit enter it goes into windows and then after about 15 seconds it freezed for a second then reboots.
i already scanned the complete system in safe mode for virus’ spyware and\or malware and it found a few things but no help, i also removed everything form start up and uninstalled any extra programs, all printers,and ran a registry machanic and cleaned up the registery. still no luck,, any one have any idea…please share