PSU does not turn on when PCI Express power connected

Last night I was just using my computer as usual and suddenly it turned off. Attempts to turn it back on would result in the power supply flashing and spinning half a rotation, yet the power button light on the front stayed on. This light could be turned off by holding the button (as you would to turn off the whole computer). I was not able to get the computer to turn on at all last night.

After work today I picked up a new power supply (Thermaltake 450W) from the store and installed it in the computer. I connected it to the motherboard, hard drive, and graphics card (Zotac 9800GT). Same exact problem. I removed the power connector from the graphics card (6 pin PCIe) and tried again. Success!!!! I pulled out the graphics card, cleaned it up, and re-seated it. No good...same problem. Is there any chance this could be a motherboard issue, or is the card just done for? It was only about a year old. Any ideas?

Unfortunately I cannot do any further testing right now, because suddenly booting into Windows XP will cause a BSOD almost immediately. The error is Unmountable Boot Volume, presumably caused by the constantly turning the PC on and off. I'm hoping to find my Windows XP CD soon so that I can get into the recovery console and get this fixed....But could this be an indication that the problem is actually the motherboard and continuing to troubleshoot is risking other components? My windows hard drive has some stuff that would *really* suck to lose, so I'm already getting nervous.

Any ideas, thoughts, recommendations - all would be greatly appreciated! Thank you so much in advance!!