It's a small round button on the motherboard, usually near the CPU junction. Among other things, pushing it 2-3 times will permanenty clear all pram, nvram and rom settings so that everything on the mobo is set back to just like it was prior to the orginal CPU being installed at the factory.
This is always a good idea when replacing major mobo components, since the electrical properties and specs most likely will be somewhat different from the original parts....
HTH
