Mysterious forces turn PC on automatically after shutdown! Ack!

My computer will shut down windows normally, then 10 seconds later there is a clicking sound from the ABIT AN8 motherboard and the PC turns on again. This happened after an "install shield wizard" would not close and I tried to reboot from windows XP but it failed to reboot. I can "fix" it by manually switching off the power supply after shutdown.
Next day I want to start it up again, I turn the power supply (antec smartpower 500W) back on, push the power button, and the computer is stone cold dead for 5 minutes. After 5 minutes pass by, the motherboard displays a "2" and goes "click click..... click click..... click click..... click click..... click click....." and the PC lights come on and the fans start to turn, but only for a quarter second or so. Kind of like an old car that wont start. After it repeats its attempt at startup about 10 times, it finally starts up and all is well.
I can "fix" that by turning the power supply on and waiting 10 minutes before I turn the PC on, then pushing the power button will usually turn it on normally. Is this a power supply issue or a mobo issue?
Specs: ABIT AN8 mobo. Athlon X2 4200+. Antec SmartPower 500W. Windows XP home.

I had a similar problem with my family's dell... I'd try to turn it on, it'd go "click clikc click click...." for minutes, and nothing would happen.

the problem probably is your power supply. Replace it and it should start up fine.

about the first part-- sorry, can;t say i know what to do.

Then it probablty is the power supply. I'm waiting to hear back from Antec. Nice to know someone else had the same problem and fixed it, thanks!

Don't know about the slow startup problem, that may be caused by the psu, but the reboot after shutdown is almost certainly because of the failed install.

Select Control Panel, Performance & Maintenance, System, Advanced, Startup and Recovery Settings. Uncheck Autiomatically restart and press OK.

Next time you shut down you will see an error screen - this error is causing an automatic restart. Come back and tell us what the error is, although I can almost guarantee it will be caused by a fault in the software you were installing.

Nope! Thanks for the help, though. I emiled Antec and they gave me RMA info, I wonder how long it will take them to send me a new one, if the RMA warranty is still active. It has been almost 8 months since i bought it. Ah, well.

I'm sorry if this a stupid a question, but have got Wake-On-Lan turned on?

Some time ago I had a similar problem, and solved turning off the Wake-On-Lan feature.

No, nothing like that.

Or a setting to have the computer return to it's last state when it loses power? (might be set to turn back on after losing power)

edit: BTW This and the last suggested feature will be found in the BIOS.