Cajun, the same thing happened to me. I would mention, at this point, that now might not be a bad time to consider sourcing a larger heatsink. The hard part of replacing the stock heatsink has been done for you.

I any case, the stuff that Serum is talking about (Arctic Alumina) is actually a two part epoxy. I've seen it on ebay. You can find it at most computer shops. The stuff sets in about 5 minutes. If you make a mess of it, don't go making more of a mess trying to wipe up the excess while it is still wet. Before it sets up hard, which actually takes about 8 minutes or so, it will get rubbery. At this stage, you can carefully go around the stuff that you want to remove with an exacto knife and just peel it off. I'd say that at about the three mintute mark, it will get rubbery. Just because around the traces and stuff with that exacto.