Possible motherboard problem

[LivedKrad]

Recently, I came back from eating a rather good sandwich consisting of ham, cheese, and mustard, when I saw that my computer had magically powered itself down. What I naturally did was attempt to start it back up, but it was a no-go. I had previously had some trouble with the starter switch, the two wire ends weren't making contact in the wire housing. Anyway, I've been trying to figure what's wrong, and here are some things I've done:

First, I plugged the motherboard power connector into the motherboard, and attempted to start the PC by completing the starter circuit with a flathead screwdriver. The processor fan spun for about 1 and a half seconds, and did not move anymore. It did this repeatedly after doing it over again. My guess is some previous discharge? So, obviously I thought the power supply was on the fritz, but I decided to grab a paper clip and connect the green wire to a black wair on the end of the motherboard power connector, just to test and see if the power supply worked. Well, it did. The fan booted just fine and ran as a constant, additionally I hooked up the system fan and an optical drive too, they worked as well.

Can anyone offer any insight?
System: Old socket A motherboard, some SiS 735 chipset I believe. AMD (XP?) 1.1GHz processor, probably 133MHz bus, but it supports up to 266MHz. Two DIMMs, two slots for (UDRAM I'm guessing?). AGP, standard ISAs, couple of IDE inputs, and some other crap I don't know. (AMIBIOS ROM setup, and I also tried resetting the CMOS, did nothing).

Any help is appreciated.

Edit: ATX form factor.

[Lenny]

Seems like a PSU problem.  You can't rule it out yet just because in can power other components.  The PSU might have deteriorated to a point where it is unable to power all the components simultaneously.  The best thing to do would be switching with another PSU.  If the problem persists (assuming there is only 1 problem), then the PSU can be ruled out.

Are there any signs that the other devices are powering up when you complete the circuit.  You may only be noticing the fan because it would stand out far more than most other devices when powering up.

[Topaz]

You might've knocked the heatsink loose and burned part of the motherboard up.

[shout]

Unplug all devices. This includes video card, ram, and processer. Disconnect all drives (power and data). Disconnect all case wires, replace and boot.

Remember that gravity is a force always working against the effeincey of your heatsink. Make sure that muthafucka is on tight. Gravity will also work against heavier video cards, eventully slightly unseating them from their homes. I have known motherboards not to power on when something is not hooked up right, instead of just beeping like any normal computer.

If you still have no booting, remove all devices again. Try to power on using a screwdriver. If this does not work, replace PSU with another one (I'm sure you can find a spare) and try again. This should tell you if it is the mother board of the PSU.

[LivedKrad]

I think I put in my post that I used a screwdriver and two separate PSUs. If not, yes, I've tested two PSUs with it and it has the same problem.

[shout]

Remove everything and try to boot it.