Computer will not power off

I have a Win98 system which will not power down and stay powered off unless the power switch on the power supply is turned off. If the power switch at the power supply is turned off and then turned back on the computer will remain powered off. Allother ways to power off the computer will immediately power back up. When you press and hold the power button it will power off, but when you release this button the computer will power back up. When you power down using windows START TURN COMPUTER OFF method the computer powers off, then immediately powers back up. I have changed the fast boot option in the registry and nothing works. Is this a hardware or software problem?