Air Filter for Computer?

[LuckOfTheDraw]

So I didn't have much to do today and I decided to clean. I got down and dirty, dusted and vacuumed almost everything I could think of. I decided to clean out the inside of my computer, which I hadn't opened up in maybe 8 months. It was extremely dusty inside, which I'm sure doesn't help with heat disersion. So, I'm thinking of making an air filter. I plan on rigging it all up myself, but I'm not sure where to start. Has anyone attempted this before? Anyone know of any particular type of filter I should look for that's good for getting dust?