Although you've told us NOTHING about your computer, (make and model, age of computer, operating system used, etc.), it would be best to start by unplugging the computer from power, taking the case cover off the computer, and use compressed air to blow out the dust from around the power supply, the processor and heatsink, and all fans and vents. Once that's done, hook everything back up again and run the computer.

If that doesn't help, then post back to this thread with ALL of your computer information so we can help more.

Hope this helps.

Grif