well, the best solution is to reapply the thermal grease so that it stops overheating. If that doesn't help, get a cheap (but better than what you are using) aftermarket cooler.
anyways, to your question. Underclocking is possible. For a board that old, you may have to move some jumpers around on the board to set the FSB to 166 (look in motherboard manual for that kind of info). If you have a newer board, you can do the same thing in bios (once again, look in motherboard manual for the exact place to find such settings). I would still recommend you get to the source of the problem and fix the cooler though.