Alright... steps on how to clean a computer (desktop) properly:
Unplug the power cord and press the I/O switch if it has one
unplug everything else from it
Take it outside (very, very, very important!!!! - who the hell wants dust in their room??)
take off both side covers and wipe them down with a clean wrag (once dry, once wet. use 2 diff wrags - let the sides air dry)
Use canned air, or a compressor or a portable air tank (cheaper is the air tank / compressor in the long run)
Take out the video card (1x screw usually) - there is a locking pin at the bottom if it's a pci-e, hold the pin out and pull up on the video card a little and the video card should *gently* pull out. make sure you take out the +12v 4pin square plug from it as well. Set this on a piece of wood / cardboard, or if you have a static proof bag... use that instead ***DO NOT PUT THIS ON STYROFOAM OR A CARPET*** (or those rags you have laying around)
After your video card(s) is/are out of the box, use the compressed air on it thoroughly. if it has dust bunnies that you can't get to, use a pair of tweezers or a q-tip (not advised) to pull it out
blow everything else out in the computer ... including the ram, cpu heatsink / fan, power supply fan (very thoroughly), around hard drives, the front and rear fans, bottom/top/side fans if you have them. Basically... you want to be able to breath into the box and not sneeze (not literally).
Make sure you wipe the bottom for dust with a clean rag, you can put some windex on the bottom of the rag after you have no huge dust bunnies in the bottom of the case.
After this, put your video card(s) back in - plug the 12v 4pin square adapter into it and screw it back in and put the sides back on
bring it back upstairs and plug it up, press the i/o switch back, power it up
notes: an old toothbrush works wonders on stains or dust clogs and heatsinks / fanblades