Computer restarts automatically

My computer restarts without giving any error or warning, not even blue screen. For a month i have been trying to solve this problem in one of the computer in my lab. It has windows 2000 OS. Local disk manager shows status of the disk drive as ‘Healthy (System)’.

I have 20 GB disk as primary drive and 80 GB disk as secondary drive. Recently i added 256 MB RAM to the comp. After that this problem started.

I have removed the newly added RAM but problem still exist. I have even reformated primary drive but of no use. Can anyone help me out of this. Any help or advise or suggestion is highly appreciated.