Sounds like it may be time to open it up and clean the dust out. You may also need to clean the vents out and replace the thermal paste. Please give us the last three figures of your serial number so we can get you to the correct guides.

If you find there is no dust buildup, you maybe infected with some Malware, Peer to Peer or Internet backup connection that is sapping your system.

After a fresh restart (cold boot) check to see if anything is consuming a lot of CPU time or network access all of the time when the system fans go into high gear. Monitor what you are doing like connecting to the Internet etc... that will clue you in on what maybe going on.

There are quite a few system resource tools in Mac OS but you need to know the difference between a good system and your questionable system that is normally running. In any case each system still has differences so you'll need have to have your wits to isolate down what is using up your systems resources.

In any case turn off any Peer services and any internet backup service. Does the problem go away?

You may want to bring your system to an Apple store as they have other tools that might isolate down the cause. There was a recall for the video logic on some systems that could also explain your problem (free repair).