I just built a new PC, my fourth one so I have some experience and I am also in the tech field but I cannot seem to find the issue for this problem.
Small tasks such as watching videos, and browsing the internet work fine but when I run certain apps, my computer freezes at the current screen. Alt-tab does not work, ctrl-alt-del does not either. I've let it sit in this frozen state for at least 30 mins and still nothing.
At first, I thought it was games specifically as I've played Guild Wars 2 and League of Legends and I would freeze, particularly near loading parts. I then noticed that the same thing happens during emulators and Microsoft Word.
I've ran a lot of tests when my PC was finished, memtest86 (at least 4 hours, 2 passes, 0 fails), furmark (multiple 15 minute tests, no freezes), Prime95 (4 hours, no fails) and do no think it is an HD issue as GW2 is on my SSD and LoL is on my Secondary HD. I've even tried both my RAM sticks separately and they both function without issue.
Temperatures look fine at 30 Degrees Celsius for my CPU and 25-30 for my GPU on Idle.
I've reinstalled Windows 7 and I am still having the same issues, currently I am getting another copy of Windows 7 incase the other copy was bad and I will install Ubuntu to see if the problems also occur.
Also, I've wondered if the PSU is the issue. I've used a PSU Calculator and 600 Watt PSU should be more than enough but the PSU I have is the only part refurbished.
So I reinstalled a different copy of Windows 7 and I am still having the same problem, installed Ubuntu too to make sure. However, when I used Ubuntu, at first the trial disk worked while the installation did not. I reinstalled Ubuntu without installing updates and I was able to boot successfully but when I installed the graphic drivers, I was not able to boot into Ubuntu. I also noticed on Windows 7, when doing the System Rating, it would freeze when testing DirectX. The GPU isn't the problem though as I've switched to a GTX 570 from a previous build and the same issue occurs.
I am wondering if this is a Mobo or PSU issue now.
So my I found a fix to the problem... Yay.....
By disabling 1 of my 4 cores, I did not encounter the freezing issue during the Windows System Rating. I got the idea from an answer for another problem where someone was having Wattage issues between the CPU and the mobo (the mobo only supported 95W max and the CPU needed 125W).
I don't know if I am in the same situation but my CPU is listed under the supported CPU list for this mobo so I am not sure if I have a faulty mobo. Also, after reading some reviews, I noticed people receiving these boards with crushed CPU pins. Not before I open my PC again, is there a chance that my problem is a crushed pin?
That is a possibility. It seems strange that you needed to disable a core to get it to behave, I'm wondering if it's not a PSU issue? I'll have to remember that in the future. I guess it wouldn't hurt to check for bent/damaged pins, but if you can't find any, then it could be the PSU since you said it was refurbished.