Keyboard is acting like a key is stucked…

I am facing a strange problem with a computer (win 7 64, s775, intel core 2 duo, 8 GB ram ddr3 1333) .
After some time of working with it windows is acting like a key is stucked to a number or letter. For example: opening start menu and the search box is getting filled with a number continusly,
I mean that it may happen on startup or even after 2 or 3 hours of work.
I change the keyboard and the mouse but the problem remains.
I ‘ve also format the pc and after the new installation the problem still remains.
Varius AV programs found nothing…
I removed the battery for 5 minutes and the problem remains…..
Any ideas???