I have an idea...
1: Take ALL of the fans out of the computer.
2: Get a plastic tub large enough to put the computer in it.
3: Put the computer in it.
4: Fill the plastic tub with mineral oil.*
Voila! Your computer is now completely silent and you don't have to worry about dust!
*Submerging your computer in mineral oil may necessitate the use of an external CD drive.