PSU Problem

[imported_sphinx]

I have an old HP computer with a 500 mhz Celeron running Win 98 se. The power supply keeps turning on 5 minutes after I have shutdown my computer. The PC does not boot or anything but everything powers on and the lights turn on (hd, cd-rom, floppy, and power). Could this be my power supply or something else. Any help will be greatly appreciated. TIA

P.S. It is a 200 watt micro-atx PSU

 

[montag451]

is there an isolating switch on the back of the pc?
If there is, see if it carries on starting up.

Are you sure you are shutting down the system? Not putting it into sleep or hibernation?