As old as it is, the thermal paste likely needs to be replaced. Also make sure the fins of your CPU cooler aren't clogged with dust and that the fan is still running well. If the fan is binding, it may be time for a new cooler.
To remove the old thermal paste, I use 91% isopropyl alcohol (the 70% stuff has too much water in it). Clean off both the cooler and the CPU. Apply a rice-grain sized amount of new paste in the center of the CPU, then re-attach the cooler; its pressure will spread the paste. I still typically use Arctic Silver AS-5.
Open up your case and check that the fan on your CPU cooler is working. Blow out all the dust from your PC with a can of compressed air. See if that helps. If it doesn't, go to BIOS and set your fan speeds to 100%. If there's still no change, remove your CPU cooler, wipe off the old paste with some q-tips and >90% isopropyl alcohol, then apply some new paste and attach the cooler.