A lot of us like kittens, they like to play and run around and chase things - just earlier my kitten was sitting in the screen doorway watching a squirrel, it was pretty funny.
They also like fans on computers. Things moving! Oh my god! So they stick their little claws in there trying to kill your fans. Try to keep them from doing this, the ends of their claws can splinter and while it may not hurt them very much (at least it doesn't seem to, he just keeps doing it) one thing you don't want is little bits of claw firing into your system.
Recently my computer started making a lot of noise from the fans. I figured that it was the heat, it is July after all. However I noticed that the fan on the side of my case wasn't working, and the CPU fan was working extra hard - way harder than normal.
I decided to open up and take a look. Sure enough the case fan had enough fur around the spinning part to slow it down to the stopping point, but here's what was worse - the CPU fan:
That is only about 10% dust!