how can i stop my computer from restarting automatically

Say something about your PC configuration and some details about your problem.

Did you install some malware ? Is your computer over-heating ? Does the same problem presist when you booted into another OS ?

tirtha87 had recently posted having problems with the desktop icons disappearing and reappearing and now(assuming its the same computer) the restarts. It might be a vundo infection and/or some more malware might have been downloaded to the comp. If its not a hardware problem(clean the ram maybe), post a HJT log, then try running VundoFix

But some more info, like an error message or something, would be helpful.

First thing, scan your computer. Try using system in safe mode.
How To Do Effective System Scanning