If your fans are dusty, you might want to hold one of the blades before using compressed air (my favorite cleaning method also). You don't want to over-rev one of your fans and damage it.
Also check your heatsink. Manytimes if your fan blades have become dusty then your heatsink has chunks of dust between its cooling fins (partially hidden by the mounted fan) also. I've improved my CPU temps 6c by removing the dust imbedded in the CPU heatsink.