Dusty tower!

[Salsito]

The fact of the matter is you will always get dust inside, the fan/s will draw it in. Obviously the greater the amount of dust in the environment where the PC is kept then there will be more dust inside. The very reason the datacentres I work in are "air filtered" to remove as much as possible.

If you can't open the PC to clean it out then you are stuck in terms of removing the gunk that has already accumulated. However as others have expressed changing the environment where the PC is kept to a less dusty one will help keep it down.