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).

Did I say that they could run at the same time on one core? Nope. I just said that OS handles which thread is driven and when by the cores.

Thread sure can freeze but OS should take care that it won't give all the CPU time to that thread. On which OS can you freeze the whole system with just one frozen thread? I'd like to know that. Even on high priority threads that should not happen. The operation of the whole OS should just slow down, but never freeze.

When the whole PC crashes it is usually caused by faulty drivers, overheating or voltage peaks (or faulty OS...), not by a frozen thread (it might seem like some thread is frozen because your GPU or motherboard drivers have crashed).