It is amazing how well desktop PCs work even when choked by dust. This one was working fine:
It is a little hard to see from the picture, but the mass of dust on the right is actually a graphics card. The graphics card has its own fan, which was so dust-choked that the the space between the blades was filled.
Running PCs in this state is not a good idea. If your machine is not working properly or overheating, it is worth a look. If it is working fine, perhaps it is a worth a look anyway. The best way to clear it is with one of those air duster aerosols.