actually you can and i have done it... <A HREF="http://www.thetechzone.com/articles/p3_heatsink_remove2.htm" target="_new">here</A> is one way to do it (dont go to first page of article... its a dangerous way to remove it).. pretty much, all you do is take a screw (the ones from your case are a good size) and place it under one of those holes (there are 4 on a p3). Then you apply some pressure and one of the legs should pop out... repeat for all four. (the pop WILL scare the [-peep-] out of you because i know it did for me...) anyway good luck.
Word of caution: I did this and it worked for me. I assume no liability if it breaks or something. The processor DID work after i replaced the heatsink.
Jesus may save, but <i>I</i> invest.