AT style or ATX style power plug on the mobo??
does it have 2 seperate plugs or does it have a shape coded single plug? the older AT style has 2 seperate plugs with like 5 or 6 pins on them that plug onto the mobo.. the ATX has a single connector and it's got different shapes at the pinholes so you can't plug it in wrong. If you've got an AT case:
you may be able to get around this by buying a new PSU and riggin an ATX power button and all of that other stuff, however you should probably just buy a new case, they're cheap. You can't just get a new PSU, the AT style power supplies also use a different way of powering on.. so the power button would have to be changed too
*IF* this is what you are talking about, u didn't make it terribly clear.