can't install anything/nsis error/corrupted files

So let me explain my story.. it's a long one I have no idea what happened but suddenly I started to get this error out of no where, I've did some research and it could have been the ethernet cable but I switched to wireless and the problem still persists. I don't have this error on my laptop so I know it can't be my net. I bought this desktop which was entirely built by scratch, and after I got it - I seem not be able to get any downloads done. I was able to download flash and chrome that came with it luckily installed, I tried to install firefox/chrome alone but no success (corrupt files). I have BSOD with 5-6 different things but I flashed the BIOS in order to reset the settings. (somehow my CPU was overclocked which probably caused the instability, not sure but I remember I had BSOD's of USB/Memory/SERVER etc. I then began to uninstall everything which surprisingly somehow fixed the issue temporarily and I was able to download once again, thinking the issue would be resolved.. but few hours later, the error has returned and I can't open launchers, extract files from zipes or basically do anything with the file after I download. I'm pretty sure my computer is safe as I'm using MSE/Malwarebytes but still, I have never persisted such problems before. Anyone have any idea what it could be? I could always opt for a reinstall but I can't do that at the moment as I don't have the win7/key disk. I have asked the local store for it incase such problems occured which I would need to reinstall in future if problems like this occur. Could this be a hardware issue or OS problem? anyone have any idea's - I woudl appreciate it and it's really annoying to deal with. I also have trouble downloading things through clients (such a s steam) or even MSE updates, it's like something is blocking it off. I've tried using other browsers like IE, same problem. I have no idea how this problem even begain in the first place..

Okay so more problems arrived, appcrashes, bsods etc. explorer.exe crashing on start and everything is literally done for.. I have no idea what to do except go back to the store and ask them to check what's going on. if I had the OS disc, I would be able to do try experiment myself but I can't. Even the internet on that desktop doesn't work anymore but a connection is still made. I feel like a virus is on there but I really really doubt it. Anyone have any solutions?

I have SP1 already. This is really frustrating, it's even starting to lag and I even BSOD during safemode. I could just opt for a reinstall of W7 but I need to head back to the store. The computer is literally getting worst and worst as the time goes on, more issues happening with different BSOD's. Could this be a hardware issue? I don't have any disks either atm to burn a W7 disk, that needs to wait til Monday, I have no idea how all these issues arrived after getting my new computer. The computer was built by a local store from scratch, they installed the OS so.

so after a fresh install, still doesnt work.. i've downloaded nvidia driver and firefox setup on my USB and transferred it to my desktop and it worked fine, seems like its something related to my hardware/desktop for sure.

Attachment 324632

this is my sfc scan log, anyone can take a look? really desperate for help guys.