I had that KIND of problem cause of a virus...
You did the system scan as i did but sometimes the virus deletes itself after doing its damage.
Mine screwed up my comp because it corrupted my modules in teh Windows category so when i started any program it got BSOD and after 3-4 days when i started windows it said:
File C:/Windows/system.32(something) is corrupted.
If you get that message then youre screwed and you need to reinstall the Windows.
Also before you reinstall i STRONGLY advise you to get drivers to your comp cause when you do clean install do windows it doesnt have anything on it.
(it takes about 24 hours to get windows working again)
But as you said you dont know youre PC stats (i assume that those are video accelerator, processor etc. ) then i guess youre gonna have to give it to some guys who know this stuff.
This may(im not sure) be the probable cause or you just have damaged the computer from outside by hitting it with something thus dealing damage to hard drive etc. vital component...