Ok, so I've been trying to build my first PC, but since I'm a noob I've had a hell of a time getting it right. My first mobo was defective, so I finally get a new one this week, only to find that I can't quite get it to work. This is the problem: when I hook it up to a power supply and turn on the power switch in the back, it turns on automatically for a few seconds, then goes into hibernate mode. The mobo led is on, but the fans, video card, processor fan all turn off after a few seconds. It shouldn't even be turning on at this point, so I think it's a wiring problem. But let me know. I'll post the specs shortly.

The front power switch will stay on the on position until you push it to the off position unless you shut down properly, at least for some PC cases and perhaps yours. If so, as soon as you plug it back in your PC has power.

When you installed the motherboard, did you use standoffs? They should have been supplied with the case. If not used, the motherboard will short to the case. Also, the rear I/O panel and installed PCI cards won't line up correctly.

Edit: for spelling error.

I hadn't foreseen the power button problem. I'll check that out. And, yes, I did use standoffs for it--I've seen my friends crucify their mobos and cry afterwards.

That was for old AT mainboards, not ATX.

And the mainboard will die, probably taking a lot of components with it.

I understand what you are saying but I think under some circumstances that could still happen with ATX boards and it might apply here (maybe). The circumstance I'm thinking about here would be before the OS is installed in a new build.

I once made the mistake of installing a PCI sound card on a new build before disabling the onboard sound in the BIOS. When I tried to start the PC for the first time, it wouldn't post due to the conflict between the two boards. Sometime during initial troubleshooting, I turned the rocker switch on the back of my power supply to the off position (I also unplugged it later). I didn't touch the front power button. It was left "on." I finally figured it out and removed the PCI card. I plugged the power supply back in and as soon as I turned the rocker switch back on, the PC turn on and started to post. Success, hurray. However, I wasn't ready to continue at that moment so I hit the front power button to shut it back down.

However, the point is under certain circumstances, I believe the system can turn on when you plug the power supply back in and without there being a problem with the system itself.

Check if your motherboard needs a four pin power connector (looks square)....and see if you plugged it in or not

Another thing to check if you can, one of the BIOS settings for handling "what the system will do if the power switch is pressed" is hybernate - if the power switch is "stuck", then the above situation makes perfect sense.

Dissconnect the case-to-motherboard ON switch and see if the system still "self-powers" to hybernate mode.
you might also consider testing the switch with an Ohm meter (or DMM) and check that it is at infinite ohms when you are NOT pressing it and has a close to 0 (zero) ohms when it is depressed. If it is always 0 ohms, it is stuck, and the shop that sold it to you needs to replace the switch..or case.

hope this helps