But the P4 doesn't have an accessible core as such, much like the later AMD K6es (you could remove the heat spreader with a razor blade and hammer, but don't try it on a P4!) Just put a little paste in the middle of the chip and stick the heatsink on. I guess you could use whatever goop comes with the heatsink, but most people will recommend that you ditch that and get some better stuff like AS3 or Coolermaster compound.