If you're having trouble with your Mac and think the cause could be hardware-related, you can run the Apple Hardware Test. It's a set of tools by Apple which perform basic and extensive tests on your hardware to see whether they are still okay. These tools are available for anyone with a Mac, and are either preinstalled on your system, or available through your OS'es installation discs.

Applies usually to older models of Mac.

You can download the AHT disc images for a number of older Mac models through here: http://www.info.apple.com/support/aht.html.

Insert the Apple Hardware Test disc into the CD drive.

Upon starting/restarting your Mac, immediately press and hold C on your keyboard. K  ...