If your fan starts running very soon after start - I would definitely suspect dust being a cause of overheating.
I have a Compaq Presario 1700 (2001 Model) and I located and Downloaded a copy of the Service Manual.
I was given the Laptop as it wasn't going and I have been able to overhaul and get it totally useable.
I completely stripped it down at least 4 or 5 times during this exercise and replaced the keyboard with a second hand unit I purchased from Singapore(B key was missing).
I upgraded RAM
, renewed the HDD
, renewed the battery.
On this model, there are 2 screws underneath which hold the plastic cover which has a cooling grille in it and covers the Start and 6 other buttons.
Once this cover is lifted out, the keyboard can be removed. You lift the keyboard a little at the back, and then move it towards the back (away from you) as there are a couple of flanges that locate the keyboard under the front cover.
You can then carefully lay it on the front section of the Laptop as it will still be connected to the Mother board by a ribbon tape.
This should allow you to access the cooling fan and clean it out.
Don't touch any of the components with your fingers or metal objects. If you don't feel competent to carry out this job - it would be better to pay someone to do it.
Best of luck. Neil.