ballen258

It'll be somewhere in the BIOS, where exactly depends on the motherboard itself. As for performance it's going to be about 60-70% of what it was in multi-threaded work and single threaded performance won't be affected unless you use the overhead you gain back to increase clock speeds.

You can do it in the bios on almost all boards. Or disable the cores in windows.
 
It depends on the program, but its normally a gain in performnace for almost all apps, or no change. Its normally a 30% peak improvement.