Keeping the inards dust free is vital on a pc. Especially if Fans get blocked.
A small paint brush and a thin nozzle end Vac to suck up the dust is ideal but you have to be "VERY CAREFUL" using the Vac inside hte pc. Suck up a jumper cap or knock a transitor could be a killer to the system.
You can get a can of Aire Spray and blow the dust out.
Main place apart from the bottom of the case having hair balls is the CPU area and memory down the edge if you happy with removing components it will make your job easier.
If the build up of dust in fans is compacted dust then replace the fans.
Also check the fans are spinning especially if theres one on the AGP Graphics Card also.
I could right a book but keep it simple. Get the dust out.
Any loose dangling cables tie up. Elastic Bands are a waste of time, get some geniune Cable Ties.