Hi

Buy a cheap long thin (but not too thin) steak knife with a finely serrated edge.
Carefully heat the top 1 cm or so of the blade in the flame of a candle (DO NOT overheat the blade, just raise it's temperature).
Gently slide the heated blade tip into the gap between the heat sink base and the CPU.
Repeat the above procedure on each side of the heat sink - pressing inwards a little more each time.

The heat of the blade and its serrated edge should be enough to nibble away the heat sink compound.

I suggest that you avoid using chemicals because it can often lead to unwanted messy results.

Regards

Chris

The best way is not do any what people have suggested. The best way is turn on the computer and run a program that stresses the processor. This should heat up the processor. Then powered down the computer. While the heat sink is hot, try removing it. If the fan on the heat sink has failed, you can use a hair dryer or a heat gun to heat up the heat sink. The reason why this works is the paste turns into a liquid in hot temperatures and becomes a solid as it cools.