Sorry if somebody has already reported a bug like this here, to be honest, I didn't read the whole thread.
So my problem is that avast! 7 slowed down my PC so it was totally unusable. (I installed it from scratch.) It consumed all of my RAM and it made my HDD work constantly. Neither Windows Task Manager and Process Explorer could show which process made that, but I suspected avast!
I had to uninstall it and install v6.