After installing the OS I've also installed the drivers, then Counter Strike (a different version then the one that was before installing the OS).
I entered the game, configured me video, audio and player settings in the game, entered a server, played aprox. 7 minutes and the PC has blocked.
After installing the OS no old data was transferred on the HDD... it was clean after a good formatting, only OS, Drivers and CS...
Now I understand that I didn't chose the right topic for the post... I guess my OS is fine and the problem is in my hardware, but I don't know what it could be.