Have you tried restarting? If not, that's the first thing you can do.
Second, try repairing permissions. I don't know if it will help, but it's worth a shot. Open Disk Utility in the Utilities folder (in Applicatiosn) and hit the "repair permissions" button.
Otherwise:
If no one else comes up with an answer, the last resort is to reinstall your system. An Archive and Install will be the most painless and get you back up and running the quickest. Make sure to check the box that says "save user and network settings" or however it is worded. But wait on a better answer. This is a last resort to "recurring problems" that otherwise don't have a fix.