You will probably have to open the case to do a good job as there may be other dust bunnies in there to clean out- like around the CPU heatsink, etc. Use canned air and a vacuum cleaner. Use the canned air to blow the dust out and in the general direction of the vacuum cleaner nozzle. You can also get an attachment kit that will work with most vacuums to clean in small spaces. Dirt Devil's kit is a nice one but I've only seen it at specialty vacuum stores or direct from the company. I haven't looked for it on the web as shipping costs too much these days to mail order a $10. item.
. I think I saw a kit at WalMart the other day, but it's not as nice as the Dirt Devil kit.
.bh.