If it's a desktop go ahead and open it up, Then blow it out using a can of pressurized air or a air compressor. You shouldn't really need to unplug/remove anything (Of course you'll want to un-plug it from the wall) unless it's really, really dirty or everything's hard to get to.
Don't use water, oil or any other sort of liquid.
If it's a laptop I wouldn't take it apart unless you're looking for one of the most pain-in-the-ass reassembly projects of your life.