problems with games that have vac security

[boutta pound this mcdouble]

ok, this started to happen a little while back but i didn't think anything of it, but now its not allowing me to play any of my games like team fortress 2, or garrys mod. what its doing is the game loads up and then as soon as i try to join a server it freezes my whole computer, forcing me to shut it off, I'm wondering if there is a simple solution, or if someone else is having this problem too, my mac is a mac book air, version 10.7.5

[Depressed Noodles]

i dont know