Start from the original system installation DVD by holding down the "c" key at startup and go to the Utilities menu where you'll find a harddrive utility. Use that to repair permissions on your system harddrive and also to repair the drive. Use the latter at least twice.
If that doesn't help, something's seriously messed up on your system. Should that be the case, simply reinstall Mac OS X and bring back your files from a backup.
Mac user since 1987. Running Mac OS X 10.8 Mountain Lion on a MacBook Air 11" & an iMac 27" and whatever's newest for my iPhone 4s, iPad 3 and AppleTV 2.
Apple Certified System Administrator 10.6, Apple Sales Professional 2008-2011, Apple Certified Mac Technician.