Installed an OS but after a couple of days,my computer restarts over and over ag

I installed a window 7 ultimate, 32bit on my dell xps M1210 about 5 consecutive times. This was what happened, when ever i install the OS, after some few days, the computer restarts itself repeatedly and doesn't even get to the desktop. This process continues over and over again until i run a new windows over and then access my old windows. Kindly help me by explaining what causes this and what i can do to remedy this catastrophe...