It would sure help if we knew more about the computer...Operating system? Etc?

First, take the cover off the computer tower and blow out all the dust and junk with compressed air..Heat related issue could be causing the restarts.

Next, follow all of the procedures in the links below to eliminate unnecessary start up programs and services and to eliminate junk files. It may take a while to perform all the steps but it will help the computer run smoother.

For Windows 2000/XP, ignore the first couple of sentences about checking "resources" and the "Performance" tab in the link below but follow the rest of the procedures including the "Services" links at the bottom.

How To Shut Down Unnecessary Start Up Programs

Because the links for the "Services" are no longer functioning in the link above, use this one:

Services Guide For Windows XP

How To Clean Unwanted Files From The Hard Drive

If you have Windows XP Service Pack 2 installed, then tidy up a little using the link below:

Cleanup After SP2 Installation
http://aumha.org/win5/a/sp2faq.php#after

After following all the procedures above, IF you are using Windows 2000 or XP, then please run Error Checking/Chkdsk and Defrag to optimize the hard drive:

How To Run 'Error Checking/Chkdsk' On Win2000/XP

Restart the computer after all of this and when you're ready to run "known,clean " video files or games, temporarily turn off your antivirus program to release those resources.

Hope this helps.

Grif