Here's a section from the Arctic Silver instructions on removal - website URL is at the foot of the post.
Removal Instructions (From Hardware)
1. Arctic Silver thermal compound can easily be removed from hardware using the proper cleaners and tools. For general clean-up, a cloth or paper towel will work well. Intricate cleaning can be accomplished with Q-tip swabs. An old toothbrush can often get the compound out of crevices that other tools cannot reach.
The recommended cleaners are:
CPU Core:
Use high-purity isopropyl alcohol or acetone and a bit of careful rubbing.
Do not use nail polish remover as it contains fragrance oils and other contaminants.
(If you use acetone, do a final cleaning with isopropyl alcohol.)
Heatsink:
Use xylene based products (Goof Off, some carburetor cleaners and many brake cleaners.) or mineral spirits.
Remember:
Once you have applied a thermal grease or melted a thermal pad onto a heatsink, it is impossible to remove all of the grease or pad from the microscopic valleys in the heatsink using standard cleaning chemicals and paper or fabric towels. Any subsequent thermal material will be applied over the remnants of the original material.
Never use any oil or petroleum based cleaners (WD-40, citrus based grease removers and many automotive degreasers) on the base of a heatsink. The oil, which is engineered to not evaporate, will fill in the microscopic valleys in the metal and significantly reduce the effectiveness of any subsequently applied thermal compound.
http://www.arcticsilver.com/arctic_silver_instructions.htm
Hope this helps!!!
