Run Disk Utility from the Utilites folder.. and "Verify" and "Repair Permissions" on the hard drive. This may cure it. Suggest you make sure all your important files are backed up. Download ONYX free from Titanium Software.. great repair utility that will keep all your files tidy and might fix the problem if Disk Utility does not.
May need to re-install OSX..
Good luck,
Jimbo