Power, no post, but boots on reset

Hello,

A strange problem. I press power, fan turns on, hard drive and CD turn on, but no post, no boot. But if I push reset a few times, it boots, and runs fine all day long.

I opened the case, and it was VERY dusty. i blew it out, re-seated the RAM and video card, but that didn't help. I moved the RAM around, swapped them out, but the problem remains. Removing the RAM entirely gives a continuous beep.

I have access to some older PSUs at work, so I've thought about testing one to see if that's the problem, but I'm not too sure if they're compatible...

Thanks!