Well, it seems that your SATA cable & hard drive are in good order.
So, just my opinion, I would suspect memory, mis-behaving drivers, running software conflict, or a virus/trojan/etc.
^You need to do this^
I know it is an overwhelming process, if you have never done it before, but it may help lead you to a misbehaving driver, if there is one.
Also, run
memtest86 from a cd or usb stick, for at least a few interations, not just one.
Download for the
bootable iso version
Download for the
Auto-installer for USB Key (Win 9x/2k/xp/7)
Which, antivirus do you use?
You may wish to un-install it and try a different one.
Try Microsoft's, it is decent and fine while you test.
Microsoft Security Center
And, run a full scan while it is on there.
Update your drivers to the latest versions.
Especially, drivers for any I/O functions. (I/O controllers, lan drivers, wireless drivers, audio drivers, mouse and keyboard drivers, video, etc.)
Check in your
Device manager to see if there are any devices with a problem. (yellow bang triangle)
How to use the System File Checker tool to troubleshoot missing or corrupted system files on Windows Vista or on Windows 7
And, open an
administrative command prompt(window) and type this "sfc /scannow" (without the quotes & put a space between the letter "c" and "/"), then hit enter. Let it scan for any files that are corrupt or bad.
If it finds none, great, just type "exit" (without the quotes) and hit enter.
If it does find any, let it repair them or follow the instructions it gives.