Why is my computer freezing while playing TF2 and full screen videos?

Sorry, I don't have all the specs of my computer. It is fairly new, a year or two old, with windows 7. I've ran 4 or 5 programs to scan for viruses, and to defrag the computer. since about a year ago every time I viewed a video (on youtube for example) in FULL SCREEN, my computer froze and went black, looping the last half second of audio over and over until I shut it off. This did not happen immediately, but after a seemingly random amount of time after going full screen. I just recently downloaded TF2 (Team Fortress 2) from Steam, and It has the same problem. After a seemingly random amount of time it freezes and loops audio etc. Ive even played it in windowed mode and it still does this. I cant tell what's wrong, if anyone could help I'd be very appreciative, thanks!