PC restarts automatically without error message

PC restarts automatically and unexpectedly without any error messages. it mostly happen when i have opened an online game. i have already disable the Automatically Restart option, but the problem keeps on. and i have also updated my display card. i tried few other online game, the same thing happened.
The restarting time is very unstable, sometime i m fine for playing few hours. But sometime it just keep on restart few time within a min.

i m using window vista.
CPU. AMD Athlon64X2 4800+
RAM. 4GB
display. ATI RadeonHD 3600series

thank you