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This support article from Apple should help:
OS X Mountain Lion: Reset your computer’s PRAM
"A small amount of your computer’s memory, called “parameter random-access memory” or PRAM, stores certain settings in a location that OS X can access quickly. The particular settings that are stored depend on your type of Mac and the types of devices connected to it. The settings include your designated startup disk, display resolution, speaker volume, and other information.
Note: To print these instructions, open the Help Viewer’s Action pop-up menu (looks like a gear) and choose Print.
Shut down the computer.
Locate the following keys on the keyboard: Command (⌘), Option, P, and R. You will need to hold these keys down simultaneously in step 4.
Turn on the computer.
Immediately press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
Continue holding the keys down until the computer restarts, and you hear the startup sound for the second time.
Release the keys.
Resetting PRAM may change some system settings and preferences. Use System Preferences to restore your settings."