If the BIOS supports reporting temperatures then enter the BIOS screen and see what the temperatures reach in there, if they are normal then it indicates a problem with your operating system rather than a hardware issue. If they are high here also, check your other BIOS screens for voltages, usually the BIOS will warn of voltages outside the desired range. A high voltage would cause excessive heat. If you have been tweaking to over-clock or perhaps might have inadvertently adjusted the BIOS voltages or other settings I'd reset it to defaults.
If the processor considers itself too hot, it will throttle it's speed down, which means it will work with a higher utilisation, i.e. games will make the OS report 100% CPU usage more easily, so the 100% usage may be a symptom of a problem rather than the cause.