To clean out the fans after 5 years of use
I used the guide on IFIXIT for this repair. For the most part it is a very simple repair. Much of it is simply removing screws. However, the step where you remove the upper case containing the keyboard was hard to follow. There are clips (four over the CD drive, and one around the infra-red detector) holding the top part of the case down. As it was my first repair on this device, I would have needed more reassurance that what I was doing was right and not going to break something as the clips are very hard to release. In the end perseverance paid off, but I'm still not sure if they have re-clipped in on the re-assemble.
Buy a can of compressed air also so that you can efficiently clean the inside of the computer. (Be careful with it's use)