Just to add that today i was playing a video at youtube and seeing other sites at the same time. My PC crashes but i still heared the sound of the video. My Firefox was open in other tab so i couldn´t see if the video was really playing.
It should be some kind of problem with explorer.exe maybe? Graphic card? (but then i should´t be able to play games). This is really strange.
Anyway, unless someone has a brilliant idea i will format my PC next week and see what happens.
I will try to update all my drivers tomorrow. Fortunatly drivermax seems to work.