Pc keeps restarting again and again?

I tried to run memtest but cant make the pc run for a sufficient time for it to get complete so far it showed no error.also i have a software named pc probe which shows the cpu temp n the fan speed,the power n chis fan shows zero speed but when i opened the cpu case it was running fine.everything is fine except when i log in to xp it only allows me 15-20 mins.It doesn't allow me to even run a anti-virus scanner it justs runs for some time n goes off.pls help i tried calling my engineer he seemed to be a bit tight on schedule so i think i have to do it on my own