constant freezing rebooting etc

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anyone know why my computer keeps freezing randomly. heres the senario. I turn my computer on, log in, 5 minutes later while browsing something or using an application it freezes and i am forced to reboot. Also, sometimes when i reboot nothign happens which forces me to manually reboot again. Not only that, my computer will sometimes randomly restart. I scanned for viruses and spyware etc..nothing. I dont think its overheating but maybe under powered PS? this only started to happen this week.

specs. soyo dragon k7
gf4ti 4800
512 mb ram
120 gb hd
amd athlon xp 1800

 

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