Desktops forum: computer won't restart? help?

Hi. I have a weird computer problem that I haven't been able to find support for anywhere, and I have absolutely no clue what is going on.

Here's the deal. Whenever I turn off my computer, whether it's been running for an hour, or 2 weeks, it won't turn back on right away. Usually it'll turn back on after an hour or so, but lately it has been taking longer and longer. Today it took like 3 hours and I'm fed up. What is going on??

Here's what happens when I try to turn it on. The fans spin and the power light comes on. I hear some clicks, and what sounds like the hard drive spinning for 1 second. That's about it.

I've ruled out the power supply and anything else really serious, cuz when it does finally boot after a couple hours, everything works absolutely fine. At first I thought it might have something to do with it overheating, and needing to cool down, and I guess that would make the most sense, but why specifically would that cause this problem?

Does anyone have any ideas at all? I'm pretty desperate. Any help at all would be greatly appreciated. Thanks.

My computer is a:
HP Pavilion 7960
Intel Pentium 1.3 GHz
Intel 850 Chipset
ASUS mobo
GeForce FX 5200
512MB RAM

if any of that helps somehow.