Lost video output, please help

[triindiglo]

I recently built this system:

G.SKILL Sniper 8GB 1333
Intel i5 2500K
Gigabyte GA-P67X-UD3-B3
MSI GTX560 Ti Twin Frozr II
OCZ 600W ModXStream
Samsung Spinpoint F3 1TB

The first couple weeks it was working mostly OK, although there were a couple of concerning errors:

CHKDSK ran a couple of times and repaired some things
2 or 3 times the computer froze up during non-intensive tasks

I mostly dismissed these as Windows errors, as I had been installing a ton of software/drivers.

I tried doing some overclocking on the processor and video card, I clocked the i5 to a modest 4GHz, and ran Prime95 for about 2 and half hours with no errors. When I moved on to the video card, I could not find a way to adjust the voltage and was concerned that the card might be bad (it also seemed to benchmark lower than other GTX560's).

So, while attempting to get some more info on the video card, I decided to run a benchmark test with 3dmark11 (the free version). Almost immediately after I clicked start, the monitor lost its signal. At first I thought it was part of the test, but the signal did not come back.

Much to my dismay, after restarting the computer nothing showed up on POST, let alone Windows.

I have tried:

-2 different video cards besides the GTX560 (One known to be working before and after this ---- One very simple one that doesn't require a PCI-E supply)
-A different, known to be working monitor
-I found a speaker to plug in and got no beeps out of it at all (? Its possible the speaker is bad, but I doubt it)
-Cleared the CMOS
-Before all of this I had reset the BIOS to standard CPU clock speeds. I wanted to diagnose the previously mentioned errors and did this to try and better isolate the problem

And very important (IMO): Windows is still working. I can start the computer and log in when I hear the Windows start sound. I have even been able to launch a media player and play some music.

The only things I can think of are motherboard and PSU, although PSU seems unlikely since everything else seems to be working ok...
If need be I can try a different PSU, but it will be a pain in the ass and I was hoping for some other ideas first.

Thanks!!!

 

[jsc]

I have tried:
...
-I found a speaker to plug in and got no beeps out of it at all (? Its possible the speaker is bad, but I doubt it)

You plugged the speaker into the motherboard inside, right?

 

[triindiglo]

Best answer selected by triindiglo.