Is my Mobo or GPU dead?

Hello. A little while ago I was trying to figure out why my system didn't emit the one beep it's supposed to after learning that you are supposed to hear a beep. I looked in my computer and found a 2x5 connector labeled "audio" so I tried to plug it into the 4-pin area designated for the system speaker. When I booted my computer I got through POST just fine along with the log in screen. I typed in my password and after a few seconds the screen turned black/unresponsive. Also, that sound you hear when you first enter the desktop crackled and faded. I was able to get to POST just fine and the MoBo read everything in BIOS. Also, I don't think my HD is the problem because I ran CHKDSK without issues before this all happened. (And my PSU voltages are all good, so I don't think it's the PSU.) I do know that my video card was running at around 75 - 85 degrees idle before this happened which really worried me (and why I suspect it.).

What makes me believe it's the MoBo though is that when I unplugged my video card/sound card and put them back in I started getting more problems. I would turn on the computer and it wouldn't even get to POST. It would just be a black screen. Also, the power button/ reset button seem to be delayed when using them (but I might have just noticed that now, it could have always been like that.). I am thinking about giving my computer to my friend for $100 + the cost of whatever it takes to fix it (so if the MoBo/GPU had to be replaced he would do it.) That could get real expensive so I want to know what I can do before calling in the pro.

Thanks for helping.

EDIT: My MoBo is an Asus P5N-E SLI and my GPU is a 8800GTS 320gb from Evga (and my warranty has run out )

Dust in the PCI-E/PCI slots? I will check. And I couldn't reinstall the cards because I can't get to the desktop.

Just to clear things up, the only thing I have done is removed the GPU from its slot, dusted the slot, and put it back in. (Nothing with drivers). I also did this with my sound card. I can get to BIOS just fine but I still get the black screen after logging in. (I have XP to clear things up.) What should my next move be?

EDIT: Is there any BIOS settings I should look into?

I unplugged the connector to the system speaker a long time ago. I guess I can try loading BIOS defaults. My friend is going to let me borrow a crappy PCI-E card that I can swap out with my current one. (I think I will try that first before getting rid of all my BIOS settings.) He thinks that plugging something into the wrong area probably wouldn't destroy my motherboard and he says the black screen could quite possibly be a GPU problem.