win 2000 keep resetting

Hi
I have a windows 2000 machine that just keep resetting it self after logon. at first i thought is was infected with the sasser virus because it was giving me the “lsass.exe was terminated unexpectedly system will restart in 60 sec status code 128” but when i scan and patched it, nothing came up. and now every time i logon the computer just resets it’s self without a warning or error message. so i was wondering if anyone knows how to solve this?
Thank you.