Up till a few months ago, i wasnt really intrested in my comp, (untill i started searching GPUs)
I started to get more bold, from opening the case to adding fans to OCing
When i opened my case though i found HUGe dustballs. Hairdryer cleaned them up good. but to get to the tricky places (HSF) i used compressed air (can or from mini compressor) and that cleared the HSF
it never affected me but now i know my comps clean. and temps dropped 1 - 2 degrees.
Dust can be real real evil, i am still running a computer from about 1999 and it still works without any problems because i have kept it clean etc. I dont think it would have made it without a few good cleans!
I would advise against using a vaccume cleaner as that can build up static electricity which is not good for the components, i believe it can fry certain parts (not 100% sure on that frying bit but vaccume's do build up static electricity), you can buy brush kits for cleaning our your computer and try to avoid having it on the floor when ur using it.
candy is right, my brother fried his own and his friends video card ==!!!. To clean use a air compressor or air can, or a static free brush. and use dust filters to help prevent dust getting in