Overclocked you would know about because you would be doing it.
The hard drive can be heard, or an led blinking...
Can you restore your system back before the problem started?
Can you do a repair install of Windows 7 if you can't restore?
There is one program called Process Explorer you can get from this
Microsoft site. With it you can check what threads are running under the system heading which
might help. But you may not recognize what the threads are from. If you decide to go this way, we can help set it up, but after you download, go to the properties page and unblock the .zip file before you extract.
Since you have checked your system with several anti-virus programs, I suppose that option is no longer viable.
Have you tried running a System File Check. If not, open an administrative command prompt and type
SFC /scannow. Maybe it will find a bad file that it can fix. Let us know how that turns out.
To open an Administrative command prompt, hit Winkey and type cmd. Then CTRL+Shift+Enter, or go to Start Menu, Accessories, command prompt and right click for Run as Admin.