The performance drop

Started freesite games that had walked quietly in 60 fps and higher!
Today I downloaded a couple of games, which were held for a long time, and noticed that fps is low and frieze
Mafia II and State of Decay was calm in 60 fps and above, the constitutional court before it was 150 fps, and now a maximum of 120.
From dust, debris and viruses is cleaning the computer. The computer is not very.
Here iron:
Processor - amd ahlon(tm) II x4 640 3.00 GHz
Graphics card - GT 640
RAM - 4GB DDR3