Computer Fan question

I have one fan that is making some highly annoying noise, and when I opened my case today to fix it, I noticed the two extra fans were not put facing the same direction.

http://www.patik.com/tools/computer/fan.jpg

For example, in one side, the Antec logo would be facing the inside of the computer, and on the other fan, it would be facing outside.
Is it supposed to be like this? I noticed one fan is 99% covered in dirt, while the other one had no dirty at all.
I'm guessing they are supposed to be like this, so one takes air from outside and drags it in, and the other takes the air from inside and expells it.
But then again, the side which they rotate could be controlled by the cpu, so, I don´t know.

The direction in which the fan blows is not determined by the CPU. Traditionally, if you have several fans, you arrange it such that those at the front suck air into the cabinet, and those at the back, blow it out creating an airflow through the case.

You are not giving enough information to determine if what you have is correct or not.

Dirty fans are less efficient then clean ones. Fan blades should be cleaned from time to time. If your case is really cruddy, then you probably need to clean the fins on your heatsinks as well.

I have one fan in the back of the case, and the other one is at the side, not the front.
The one in the back is the one who never got actually dirty in 1 year of having the computer, the fan on the side gets dirty in a matter of days.
Since this one on the side is getting like that, I'm assuming it´s the one pulling air inside.
When I clean the small holes on the case that this side fan uses, in 1 week or so it already gets so dirty you can barely see any holes at all.

But let´s say I change so this 2 fans are now only expelling the air inside, wouldn´t that lead to overheat? Or do you think, given this conditions(1 back, 1 side), it could be more efficient, and would also prevent the dirty mess it is causing :eek:


Edit: Anyway, my computer was making much more noise with the two fans expelling air, so I changed back to normal.
Thanks for the help!