Some games freeze my system completely

This is completely wrong. A slow computer will just run slowly, that is all. No such thing as crash due to overload (assuming you don't run out of ram and have swap disabled I suppose)

It will hang the system if the OS's scheduler prioritizes it over other things like the UI (i.e., if you set the process priority to High/Realtime.) If you don't do that, something funky is going on, probably. What does Task Manager say before it freezes?