[SOLVED] PC won't boot after disconnecting power

First, I really appreciate this forum being here; I hope it turns out more fruitful than Gigabyte tech support. I'll try to provide as much information as I can.

I am reassembling an old computer for a friend. Specs are as follows:
Gigabyte GA-MA790FXT-UD5P (Dual BIOS, if that makes a difference)
4GB Crucial RAM
AMD Phenom II x3
40GB Western Digital HDD (soon to be upgraded)
NVidia 9800GTX+ Black Edition
500W PSU
Wireless Adapter
Windows 7

The PSU and Wireless Adapter are new. I tested the PSU and it works perfectly. I also have switched with another PSU that I know works because I use it every day to verify that the problem is not the PSU. The HDD is really old; it works but I intend on replacing it. All of the other parts were purchased circa March 2009. Also, the first thing I did with the rebuild was check and replace the CMOS battery.

The problem I was having was that the computer would not boot without being jumped by connecting the green and black cables on the 24-pin connector with a paper clip. With the help of Gigabyte support, I was able to load the fail-safe default settings and optimized settings for BIOS. That allowed me to reboot the computer without using the paper clip.

But that didn't solve everything. If I disconnect the computer or flip the switch on the back of the power supply (then reconnect everything, of course), the computer will not boot or respond at all. No lights come on. No fans. Nothing. It's as if it isn't receiving power.

After a month of back-and-forth with Gigabyte Tech Support, they suggested I update the BIOS from F3 to F7 or F8N. I used @BIOS to perform the update and now have F7 on the computer. That didn't help at all. The update ended in an unresponsive black screen. There was a white underscore at the top-left, as if promting a command, and a green square with an "N" next to it near the center. After 2 hours of nothing, I shut the computer off and booted it back up. The update applied itself and Windows7 loaded. I turned it off, flipped the switch on the back, waited 10 seconds, and attempted to reboot without the jumper. I got nothing.

I've been at this for over a month. Gigabyte has given up on it, saying I should RMA it but it's after warranty. I have scoured the internet for information to no avail. Please help.