problems with windows xp restarting after startup

my system kept rebooting could not get into windows in any mode. reformated and installed fresh operating system. System worked fine at first then while installing anti-virus system came up with this error WINDOWS\system32\lsass.exe -1073741819 NT AUTHORITY\SYSTEM system is shuting down. it would shut down and reboot. I was told this was the sassor worm. i reformatted and installed a fresh operating system again went and scanned for viruses in pandasoftwar and it found no viruses. i shut down my compter and rebooted it and i got a black screen after the xp logo comes on then it won't go any farther just that black screen with no cursor. tryed my reset button to see if it would work but it didn't it did the same thing. it went to the xp logo and black screen and no farther it is fine as long as i don't have to restart my compter or load up anything. i also scaned my windows xp disc for viruses and it came up with none. need a response asap please help.