Dust on the inside or outside of the PC box?
If on the inside, does the case have a fan in the front and rear?
If on the front, try cutting a piece of cloth membrane that's used to seal the top of parapet walls, roofs, cracks etc, (can get from any hardware store in or near the paint section) - cut to size and place over the front fan intake - on the opposite side of the metal chassis to where the fan fits - I use rubber bands to hold in place, but you can use velcro, prestick, double-sided tape, etc.. It works wonders for me and keeps the case very clean inside. When the membrane does get a bit blocked up, I cut another piece and replace. I also blow out any dust particles that do manage to get through, every month or two...