The Pentium 4 is as commonly known equipped with a so called IHS. IHS is an abbreviation for “Integrated Heat Spreader” and is as the name hints an integrated heatsink on the CPU. It’s placed on top of the core and makes sure that it doesn’t break easily, compared to AMD’s cores whose contacts with heatsinks are totally unprotected.
The problem with the IHS is that it hinders the thermal transfer between the core and the heatsink, since the IHS means one more contact area between materials. By removing it, you can lower temperature with a few degrees, but by doing so you also threaten to destroy the CPU, both when you remove the IHS and when you mount the heatsink.
AMD’s CPUs, however, are doing rather okay without an IHS, so in my opinion, it yet remains to see how fragile the P4 really is.
MadShrimps has written an article which describes how to remove the IHS, but unfortunately, they included neither performance nor temperature tests. It has promised to do that in the second part of the article however. If you want to remove it and test for yourselves, this seems to be a fine method to use.
Read more here.