How to replace a cooling fan for Compaq Presario

I would definitely consider taking it to a certified repair shop first.

If you must do it yourself, get the proper fan, (Which you can find on ebay for around $22 new), a set of small screwdrivers, and preferably an anti-static wrist band. You will likely need to take out every single screw in the thing to get to the fan. It will be fairly straight forward to replace unless you need to take off the heatsink, in which case the following information may be useful. If you can avoid taking off the heatsink do by all means.

As with all electronics wash your hands first, touch a ground (Typically your sink facet is grounded), and stay grounded if possible (With the wrist pad), and avoid touching any chips as the oils from your fingers can cause problems.
And always take the battery out and make sure it's not plugged in before starting anything!

The fan itself has a 3 wire connector, and will simply screw into place on the heatsink.
If you do remove the heatsink from the cpu you may want some heat transfer goo .

Some heatsinks now don't use heat transfer goo any more and have a pink pad that melts at high temps which helps transfer the heat.

I prefer arctic silver for the heat transfer goo, and to remove the old heat transfer goo before adding the new. Place a very small amount on the heatsink, enough to cover the part it makes contact with and no more, being very careful to not allow any air bubbles, and making sure it's spread evenly across the surface. Don't use your finger, use a flat edge to spread it, Maybe a box razor blade or something similar.

If it uses the pink pad, do not put heat transfer goo on as when the pad melts the transfer goo will mix with the pad and degrade the performance of heat transfer. Ensure the pink pad is in the same place it was when you removed it.

Put back together, ensuring every screw is back in, then cross your fingers and toes.