Why does my computer boot as soon as power is applied?

Just a thought (and don't be offended as sometimes it is just this obvious), but do you have the case power switch connected properly? I am assuming you have checked to make sure it is off if it is positional.

Yeah I guess I should have been more specific about "front panel" connections. I'm talking about the HD LED, Power LED, Reset and Power switch stuff.

So I have checked that stuff out and have even unplugged all of it to see if it made a difference. Unfortunately, it still did the same thing.

Thanks though.

This is a new build, did you re-use any old parts?

I've run across this with a few other machines that did this, and two now had a bad PCI card, as soon as I removed them the problem would go away. Another was a bad stick of RAM.

You might try and slowly remove one piece at a time and see if the problem goes away.

Also make sure you don't have any stray stand-offs underneath the motherboard, if you just put this board in.

isn't that a setting in the bios

Yes, check the power section in the bios. Your system might be viewing "on and off" as power failures and rebooting when you switch it on.

As a couple people suggested, it can be in the BIOS, specificaly the "Restore on AC Power Loss" option. Make sure it is set to "Power Off". Hope this fixes your problem.

Another thing to check is that the power on button at the front of the case is not sticking. I had that happen once, and it was easy to fix. I simply used a round file to enlarge and clean up the hole that the button sticks through.

Thanks for the ideas everyone.

The BIOS options sounds like a promising solution. Hopefully, that will fix the problem before I have to go and take the motherboard out to look for standoffs.

I actually thought it might be an issue with a standoff somewhere when I changed the board out a while ago. (I haven't used the computer much since then.) Hopefully it wont get to that point though.

Anyways, thanks for the help. I'll let you know how it turns out.

Well, I think that I have finally diagnosed the problem.

After checking my BIOS settings and checking for any stray stand offs to no avail, I started removing components from the computer. I pulled the graphics card out of its AGP slot and, voila, silence when I turn on the PSU switch. Push the power button and it turns on and when I pushed the Start button, U, U (thankfully I was able to remember that with no display) it turned off completely.

So now I'm wondering whether the problem lies with the graphics card or with the motherboard/AGP slot. Unfortunately, I dont have an extra graphics card lying around to test it out. Hopefully, my friend at work will be able to find an old one of his so I can test it out.

In the mean time, any ideas about a cheap AGP 8x GPU? Maybe something with passive cooling if I just need to replace the card and not the whole mobo?

Thanks again!

If you want to stick with AGP, ASUS has some mobo's left, as do others I'm sure, though AGP boards are getting harder to find. If you have to replace the card and the mobo, you might think about going to a PCI-E board. Won't be much difference in cost, might even end up cheaper, and you'll have a much wider variety of cards available.

Unfortunately, this HTPC build was supposed to be low budget. If I was to get a PCI Express card I would have to get a new mobo and processor (the processor is Socket 478).

There are only two 478 mobos on newegg that have PCIe and neither of them look particularly good.

Hopefully my friend will be able to find an old AGP card so I can at least figure out whether the problem lies with the graphics card or the motherboard. If it turns out to be the card, I wouldn't mind paying $30-40 for a cheap AGP card with passive cooling.