It's really not all that bad. Since the way you open up the case varies from computer to computer, you'll need the manual. Truth is, even without the manual, you can probably figure out how to open it just by looking. Make sure you unplug that sucker before you do anything else. Once you have the case off, look at the exhaust vent for the small fan.
Dust can clog the screen that protects the fan.
If that happens and airflow is restricted, the inside of your computer can get as hot as an oven. The chips can easily fail if you leave them half-baked. So cleaning the exhaust grille can save you money and frustration. Now that the top is open, also take a look at the circuit board itself. If so, then you may have a short circuit just waiting to happen.