Boot from the OS X install CD, run Disk Utility from the Utilities menu. Select your hard disk on the left side and repair the disk (on the First Aid tab). After repairing the disk, try repairing the disk permissions. You can also use Disk Utility to clone the disk or save an image of the drive as a disk image to an external drive to back up the data if you don't already have a backup.
If repairing the disk and permissions don't solve the problem, then try reinstalling Mac OS X over the existing installation (upgrade install). If it won't let you, then do an archive and install, which will backup all your apps and home folder to a separate folder on your hard disk and install a fresh new copy. I think you also get the option to import your settings and documents.
Mac User since 1989
MacBook Pro 15.4"/2.33GHz Core 2 Duo/4GB/250GB HD/256MB VRAM
Mac mini/2.0GHz Core 2 Duo/1GB/120GB HD
PowerMac G4 "Sawtooth"/1.4Ghz G4/1GB/2 x 120GB HD/64MB ATI Radeon 8500
iPhone 3GS 32GB on Rogers Canada
Master of the Art Of Geek.