Desktop

sounds like a virus, I would reinstall after fdisk /mbr

Might be a bad power supply too. If it's relatively inexpensive, I wouldn't beat your head against the wall too much. It's no fun doing that.

Because the PC runs in safe mode doesn't mean there isn't a power problem. In safe mode a device may not come on that otherwise would overload the power supply.

First if any, what changes have been made to the hardware or software. Is there anything different with the surroundings like another device plugged in to the same surge protector or power outlet. In otherwords start outside and work your way in. This may just be a loose power connection. After illiminating outside sources then dig into the PC.

What happens if you boot to a command prompt with a boot disk. OS doesn't matter cause you're just seeing if the PC will stay on. If it does then its a software related issue.

In safe mode uninstall drivers for modem, sound, nic, and all external peripherals connected USB, COM and LPT ports. After shutting down physically disconnect them. Only connect a regular keyboard & mouse. In the BIOS, disable hardware monitoring, the onboard devices like modem, a common cause of this problem, network card, sound, COMM and LPT ports. Sometimes there's jumpers on the mother board to disable the onboard devices if so look in the users manual for how to do it. If you have a video card disable the onboard and install that.

Start the system if it works add items one at a time until the problem occurs. If its an onboard device you can leave it disabled and use an add-on card.

If it will run indef. in Safe Mode, then I would suspect either an add in card like Video, modem, etc. or a specific program in your start up folder. Try this first in Win98SE. Start it in Safe Mode, then Start>Run>Type: msconfig>Enter

1st thing to do is click on the button "Create Backup", and follow the instructions to create a backup even if a recent 1 has been made.

(Now get out a pen & paper and make a simple chart of the 6 checkboxes below the "Selective Startup" section) You will be trying many combinations so keep track of that which you have checked out already.

Check the circle for "Selective Startup", uncheck "Load startup group items". Restart the computer. If it loads Windows normally, then it's simply a program in the start-up folder. Here's what you absolutely need to run Win98SE only, not counting your AV and other optional start-up items. If it starts normally, then go back to msconfig, re-check the "Load startup group items" box but leave it in the "Selective Startup Mode", go to the "Startup" tab up top, then uncheck all items except the following.

Scan Registry
Task Monitor
System Tray
Load Power Profile (there needs to be 2 separate entries of LPP, DO NOT uncheck either of the "Load Power Profile" items).
Scheduling Agent
Load QM- If Present (if you uncheck it, then it will just create another entry upon re-boot, not good).

Ucheck all others present. Re-start the computer and see if it starts normally (if it does then the only items in the system tray by the clock should be the Task Scheduler & Volume Icon).

Now you have determined that it is a startup item or hardware associated w/1 of those items. I would go back to the start up tab, keeping track of what I do, and recheck the boxes for those other startup items 1 at a time and restarting normally until I had a shutdown, freeze or whatever the original problem was until I found the program that causes the problem.

Bad motherboard?
Did you try upgrading the BIOS on the motherboard? If so, then get a new motherboard.