Here is my build:

Core2Q q9450 @ 3.0 Ghz, Primed and temps were low
4x1GB of DDR2 800 RAM, underclocked to DDR2 751 to get 1:1
Radeon 4850 (Asus EAH4850)
ABS Tagan BZ PipeRock 800W PSU
Arctic Cooling 7 w/the thermal paste that came with it.
NZXT Tempest
2x 500GB SATA2 HDs, NOT IN RAID
2x optical drives

So my build has been working perfectly for a little over a month now. I was playing Unreal Tournament 3, when suddenly everything just shut off. I reflexively looked at the PC as it died, and I could swear I saw a white flash in the PSU.

I was really confused. I had no idea what to do. So I unplugged power, and let it sit for an hour. Plugged it back in, and saw the mobo indicator light turn on. When I hit the power button, one of my LEDs and most of the lights on the PSU turned on for like .2 seconds.

The obvious answer would be that I have a bad PSU, but if so why can I power on for a second? And why does the mobo light turn on? What could cause this anyway? I am pretty sure I wasn't overheating. The CPU has never climbed above 58, so that is out of the question.
PLEASE HELP ME!!!

It happens more often than you would expect, so you must pay extra attention to your next PSU choice. In this particular line of product, while many PSUs are needlessly overpriced you get what you pay for. One less thing to trouble shoot if and when things go wrong.

Why is your PSU working at all you ask? For a number of reasons. Perhaps you are triggering a short or fault condition, perhaps a component has yet to fail completely. As it stands now, it is broken and potentially dangerous, get rid of it carefully, even unplugged and sitting quietly for a day one of big capacitors might have a lot of juice left in it.

The real question is how badly is your mobo, ram and CPU damaged. A dramatic PSU event like that likely generated a surge and you need to physically inspect the power leads, power sockets and pins and traces on the mobo...any discoloration or melt or bad smell or scorch or stress marks indicate more current than the delicate internal traces can handle, most of the motherboards wires you can't see, they can easily fail.

Maybe you got lucky and you can drop a new, quality PSU and all will be well for years. Maybe you need a whole new system. Good luck!

P.S. In my experience first the CPU would fail, then the RAM before the MOBO would fail from a surge of that nature, but that is usually only the case in systems that can at least boot. Non-booting systems are trickier and require a thorough inspection.

Trust your first reaction. The PSU is likely toast, at least in part. Check the troubleshooting guide on the manufacturer's website. New PSUs are switching devices, and some use separate relays for each rail. The PSU isn't necessarily monolithic.
A quick test is to swap out the PSU with a known-good unit. Also, small PSU test devices are available for around $20.
Another approach is to disconnect all components (drives, front panel accessories, all system I/O, memory, and all cards), then try to power on. In this condition, you should at least get the beep codes for your MB indicating no memory.
What MB is it, and does it have video onboard?
Good luck.

Ouch...makes me sad. Tagen seems like a good brand but I would never buy anything from them personally. Try a different brand and a higher quality psu.

You can always get a bad PSU.

However, I do want to add that just because some review gives a PSU high marks, does not mean that it was a quality review.

Only a very few sites know how to properly test PSUs. The rest of the tech sites range from "B" efforts to far worse.

PC Power and Cooling is one of the very few PSU brands that don't compromise and can be relied on. Corsair is earning a similar rep...maybe not quite there yet.

Given your specs, I would think you only need:

http://www.newegg.com/Product/Prod [...] 6817703005
or
http://www.newegg.com/Product/Prod [...] &Tpk=750TX

@warehousenorth: I have an Asus P5Q Pro, forgot to add it to first post.

So I am pretty sure it is my PSU, but I am trying to think up a way that I can test what parts of it are broken. I have disconnected everything, and then it still did the same thing. I went to Frys and got a PSU tester, but the problem is that the PSU shuts off so fast that the tester doesn't even have time to get a reading. I just hope that nothing else is wrong, that was an EXPENSIVE processor.