Noisy fan on a Compaq laptop Presario 2500.

You will need to remove the Soundbar, keypad, lcd screen, top case and the heat sink assembly for the cpu. (in that order)

There are at lest 40 tiny screws (some differently sized) to take out and a little coaxing to get the top part of the case off.

Either take it to a pro or buy a cooling pad so the fan doesn't need to turn on.

Have you tried following the instruction in the Service Manual(pdf) published by HP online?