Stuck in permanent "Reset" state?

Hi all,

A couple of months ago my PC switched off within the first few minutes of playing Portal. It wouldn't turn back on again so I figured I'd replace the power supply - the PC is now a few years old so I didn't think too much of it.

Last month it happened again, so I replaced the PSU, but now I seem to have another issue where the unit is powering up and either not progressing to the POST, or just switching itself off after a few seconds.

I thought it might be the motherboard so I also replaced that but the same thing happened. I took out the possible offending components, ie RAM, video card, unplugged all the drives etc, but still nothing changed. It wasn't until last night that I noticed that it seems be mimicking what happens when I hold the reset button - in my case, the hard drive light stays on and and I get a "no signal" message on the monitor.

It seems that replacing the PSU has resolved the initial power issue, but now I'm thinking that - and bear in mind I have very little electrical or electronic expertise - maybe there's an issue with the power/reset switches on the case or that something somewhere has shorted out?

Thanks