What's wrong with my computer!? Motherboard fried? Need help desperate

OK so this might be confusing, but PLEASE help me.

Last night my computer said my ethernet cord is unplugged. Which isn't possible because it's plugged and working fine right there. I had to clear CMOS to get everything back on track.. weird.

TODAY, I couldn't even game! I get about 5 fps in CRYSIS. and Mirror's Edge lags like CRAZY. So i thought my gpu is fried. I switched my GTX 285 to a 8800GTX and SAME THING happens. So whats wrong? I tried PRIME95 and cpu seems fine. I was copying files/extracting earlier with no trouble at all so im not concerned it's the CPU. I took 1 stick of ram out ( 2gb) and tried to boot from there. It didn't work. I get all the lights and fans working, but no BEEP. nothing on screen. BLANK. Then after about 5 seconds, my pc shuts off and reboots, then does the same thing. My moniter never displays anything.

Another that happens is sometimes, when i type, it would lag. I would type something, then2 seconds later it shows up and then spammmmmmmmmmmmmmmmmmmmmmmmmmmms the same letter... it's like my keyboard was jammed but it wasn't because it works fine on my other machine.

SO whats wrong? Fried mobo?

My conclusion: It's not GPU. It's not PSU ( all bells and whistles turns on). It's not HDD (that I know of, because if it is, it should get pass bios into xp then crash). It's not RAM ( that i know of) It's not CPU ( I think, because it was working fine?) So it's gotta be the motherboard?


Help please?

thank you

SPEC:

GTX 285 FTW
i7 920
650 GB WD HDD
2x 2GB 1600hmz DDR3