Repairing your Hard Drive:
In addition to repairing permissions, you should also attempt to repair the hard drive's filesystem if you experience crashes or other odd bugs.
You can repair non-system drives in the Disk Utility (/Applications/Utilities/Disk Utility.app) program. Select your hard drive, switch to the "First Aid" tab, and press the "Repair Disk" button.
To repair your primary "bootable" system hard drive, you must boot into single-user mode (hold the apple and 's' keys while rebooting). At the command prompt, enter the command:
This will force FSCK (FileSystem ChecK) to run and automatically repair any errors it encounters. When completed, if you see the message that your filesystem was modified, run the command again. Repeat until you receive a message that says your volume "appears to be OK." When complete, enter the command:
As an alternative to using the commands in single-user mode (ie. for the terminal-phobic), you can reboot from your Mac OS X Install CD/DVD and choose Disk Utility from the Installer menu. Then, you can use the earlier instructions to repair any hard drive including your system's boot drive.