There are many reasons why your fan is running hard. I don't like using software to over-ride what the system wants to do. It's likely you will damage your system running it like this so you do need to deal with the problem.
- Something is blocking the system to breath (keyboard covered or back vents blocked)
- Heavy dust buildup inside due to the age and/or environment being dusty
- Thermo paste has dried out and not conducting the heat as well to the heat sink
- One or more of the temp sensors or fan unit is broken or you could have a problem with your HD or the display sensor.
I would start by doing the first two things on my list as anyone can do them. Get the needed tools to open your system and a can of can'ed air and a soft brush to scrub down the fan blades heat sink fins and the logic board taking care not to snag any of the small components as you can break them off with little force. You should have a anti-static mat & wrist strap and have it properly grounded to protect your system from ESD. Using the can'ed air blow out the dust making sure not to spray any of the liquid out.
Put your system back together is it better?
If not you will need to go back in, but before you do download and install a better temperature monitoring app which accesses your systems thermo & fan sensors telling you what they are saying. As it can tell you what area needs to be looked at more carefully or you could have a bad sensor or fan unit. Here's a free app Temperature Monitor 4.85 that should work with Snow Leopard.
In any case review this iFixIt guide on removing your heat sink iFixIt Guide - Heat Sink Removal. Make sure to get a good grade of thermo paste (iFixIt sells a good one) and use the correct cleaner to remove the old.
More often than not cleaning and putting new thermo paste on the heat sink fixes these systems.
At to a bad sensor or fan lets us know what the software tells you. So we can guide you the rest of the way - Good Luck