Power Button

I have a Gateway MA P5-166 PROFESSIONAL PC. When I do a normal shut down, I get the screen that says it’s safe to shut off the PC. When I press the power button, nothing. I’ve kept it pressed for 10-20 seconds, nothing. I’ve changed the power settings both in the BIOS and within Windows. I’ve changed it to enabled and then to disabled with no results. I have changed the switch, thinking it was a bad switch, with no change. I currently run Windows 98SE. Any suggestions? I don’t belive it’s a software issue, thinking possibly motherboard. Thank you.