From LightWiki

Go to /Applications/Utilities and launch Disk Utility.

Then select your hard drive in the left column and in the First Aid tab click Repair Disk Permissions.

If that doesn't work restart and immediately hold the Apple key and S as it chimes until it begins to boot into Single-User Mode (black screen, white text) then let those keys go. Once it boots into a command line, type fsck -fy to repair any discrepencies on the boot drive. If it finds any problems it will run again automaticaly to be sure they are fixed.

Once it's done type shutdown -r now to restart.