I believe my fans need to be replaced. They are always running, and I'm scared of my computer overheating.
Fans will run constantly anyways - they will spin faster when the heat increases inside the case. If the inside of your case is dirty (dusty) it will cover the thermistor which will hold in heat; the dust will act like and insulator and hold in the heat, keeping the thermistor "hotter" telling the fans to spin faster.
Also - in summertime, when the inside of your home is warmer (and more humid) case temperatures will rise causing fans to spin faster.
My advice - get a can of compressed air and take it outside. Open the case up and blow the inside of the case getting all of the dust bunnies out of it and try that to see what you get. Do not be afraid to blow all the components (power supply, around the hard drive, CD/DVD drive, etc) get as much of the crap out that you can. It might help.