computer

Maybe the system has been configured to shut-down automatically after sometime. Or worst come worse there may be a virus.

so wat

You also need to check out your event viewer, I can't remember how to do this in WIN 98. Although go to the Control Panel, look for Adminstrative tools/systems tools and search for "Event Viewer" it should be under one of those tags, on the screen that pops up look for a red icon indicating something went wrong. Feel free to post it back here if you find anything.

your windows infected by virus or worms. scan your computer and clean it.

The proper thing is to call a software engineer and fix the problem immediately. I have faced this problems many times.