Your system is most likely restarting as a result of a STOP error.
To stop this behavior and to analyse the error, try the steps in
this article: http://aumha.org/win5/kbestop.htm
If the error indicates a file name that begins with ati, or nv (nVidia)
the most likely solution would be to install the latest drivers using
the instructions at: http://mvps.org/nibblesnbits/Video.html#drivers