Computers require regular cleaning, especially in a dusty enviroment. Buy a can of compressed air from your local computer retailer for the hard to get spots. If you use a vaccuum cleaner be very careful, you can get a kit with small brushes that connects to the vaccuum hose specialy for cleaning computers and keyboards or hard to get spots. i would say the fan is running at full speed all the time because it is probably accumulated dust which makes it overheat. open the case and clean out all the dust but make sure to earth yourself to the case to avoid static electricity.
Allen