I have 3 GHz Pentium 4, and it's not best for playing full HD videos. I noticed, that when I'm playing full HD video, only 50% of my cpu is used, so only one of virtual cores is doing it's job on full HD video and the other one is idle. Sould I turn hyperthreading off, to get all 100% of power available for film?
No, don't turn it off. HyperThreading doesn't cut the processing power available to a logical core in half. 50% of the CPU used with HT on means a logical core is fully loaded, while the other is free.