If you are using XP then the computer is likely to be somewhat old and subject to failing components.
Could be the PSU, failed thermal paste, loose connections, etc..
However, that is just a generally observation.
Update your post to include full system (pc) hardware specs.
Also, has there been any recent work done on the computer: new components, cleaning, reseatings, and so forth?
Any known new sofware installs or updates?