Some games freeze my system completely

I've never seen that happen on on ne PC's because UI usually runs in separate thread so no freezing on multicore CPUs. (Though insane lagging/ constant no response is still possible)

Just a note here: Threads have nothing to do with CPU cores, threads & processes are OS specific... Single core can run multiple threads, so no freezing on single core either (with a proper OS).

Nope. Multiple threads can run on 1 core, true, but not at the same time (without HT anyway). They will be switching places so if one freezes it can freeze PC completely (and it happened on my old PC a lot there everything froze with no response, just still image).