You can remove the fan itself from the top of the heatsink. It should have four screws holding it to the heatsink. If it is as bad as you say, you definately need to remove the fan ato clean it, as well as to blow out the fins on the heatsink.
Just be sure that you plug the fan back into the mobo when you remount it. Otherwise your CPU will last a very short time before it fries...