Computer dust filter, where can I get one?

Hi there, just wondering if anyone knows where i can get some sort of computer dust filter that would prevent dust from entering the pc case. My grandparents house is very dusty and their last computer over heated from too much dust on the inside. and rather than having to clean out the case like every month it would be much easier if there were some sort of filter i could apply to the ventilation holes so dust couldn't get in.

svc

Negative air pressure will reduce the amount of dust entering the case. Most cases are not designed for this, so there may be hotspots with higher temperatures within the case. This shouldn't be a problem for your grandparents computer though.