close down problem

you may have to reformat your hard drive then reinstall windows 98. This would ensure that the virus is not on your system at all. Also if this virus can reside in memory or your Bios you may have to remove the battery on your motherboard then put it back in. The battery that hold your bios is a set if instructions that tells your computer what to look for at boot up and holds your correct time and passwords.

mcnich, If this is a virus what triggers it? for example puter as now been on for 6 hours without problem, if it were a virus wouldn't it shut down often and at regular intervals?

I took it that you were talking about a virus but if it is not then it could be that your system is over heating. There are a couple of fans in your system. One is for your power supply another is for your CPU. If you can take the side cover off of your system and see if there are any other fans, if not you can always add one or two. One would be in the front pulling in cooler air and the other would be in the back below the power supply pulling out warm air. Most store systems like HP and etc. do not added the extra fans because of cost. About $12 each. One sure way to see if the system is over heating is by changing the temp in the room the system is in. or by running with the side cover off the computer and see if this helps. If it does get a can of air and clean out the existing fans and think about adding a system fan.