Help w/ a dysfunctional computer?

Hello! I know this is a lot of text, but I've got a lot to explain.  A friend of mine has been having issues with his PC for a while now, and he hasn't been able to get it fixed. Now he can. Anyway, it seems very laggy or delayed in the desktop. Like if I make one of those blue squares on the desktop, it lags like half a second behind. Also gaming performance isn't even close to where it should be; it couldn't even run skyrim on medium 720p. I already tested his graphics card in my i5-750 rig and it performed just fine in BF3 and Skyrim @ 1080p. We have already done a fresh install of windows, so it shouldn't be a software issue. I've done a memtest; that was not an issue either. I'm led to believe it must be the CPU, PSU, or mo/bo. The CPU temps don't seem that bad; ~44 idle and 75 at full load. We were thinking of grabbing an 1155 mo/bo, i5 3570k, and a GTX 670 for Christmas, but I would hate for the power supply to be the problem and for his PC not to work. This seems like a good power supply and it has great reviews everywhere, but idk. Also there were no problems with the screen flickering.

Is there some way to check CPU efficiency or something? Is there a way to check if the mo/bo and PSU are working properly? Are we going to have to take it to Geek Squad (I was trying to avoid this b/c I'm a DIY type of person, but safety first)?

Help would be much appreciated!

Specs of his machine:


CPU: i7 920 @ 2.66 GHz

Cooler: CM Vortex

Mo/Bo: Some fancy pancy EVGA socket 1366 mo/bo I don't know the name of

RAM: 3 x 2 gb ddr3

HDD: WD Caviar Black 1 tb

PSU: Corsair hx750w

Case: NZXT M59 Classic Series


P.S. He said that once apon a time ~1-2 years ago his CPU fan stopped working properly and it was overheating or something and so a buddy of his (the guy that built the computer actually) came over and installed that cooler master one.

P.P.S. I didn't know which forum section to put this under, but I suppose that "Build a PC" is safe.

Sounds like HDD. If possible, I would take another HDD, intall linux on it (windows isn't good with this stuff) and try to run it on his computer. If that works fine, you know what the problem is. Otherwise, I would both try using different RAM (maybe yours). If that works, put his RAM back in and run memtest and Prime95 (blend setting) for at least 24 hours.

Incase you didn't know, you don't have to unistall windows, or partition to install ubuntu on windows 7.

 i have better man. use seatool from seagate