System shutdown initiated by NT Authority/System

I booted the P.C up on Thursday evening, it all fired up as normal but once a program was attempted to run – the system gave a 60 second warning then went into a shutdown and restart sequence. The error window that popped up at the start of the 60 second warning was quite specific. I noted down what it said…

“System shutdown initiated by NT Authority/System”

“c:/winnt/system32/lsass.exe terminated unexpectedly with status code 128”
My System is Win 2000 Prof.Help Please.