I just finished a build for a heavy smoker. His old computer was disgusting with a mix of tar and dustbunnies hanging everywhere inside the computer. During that build, I discovered these little beauties:DemciFlex DemciFilters
It's a very fine mesh filter that attaches to the outside of the case and are easily removed for cleaning (they are attached with a magnetic frame).
You can purchase them at www.frozencpu.com and www.performance-pcs.com.