If it's been awhile since you've opened your case, I would do a thorough cleaning before anything else. You can use "canned air" which Fry's sells for about $8 for a 3 pack. I removed the heatsink from a friend's celeron recently, and ran it under tap water and let it dry completely before installing. I used some radio shack white thermal grease, applying a tiny dab on the center of the cpu. I would also clean the power supply fan and hardrive area. I don't know the temperature range of the pIII, but the cleaning will certainly lower the heat in your system.