Do you have the installation discs for it? If so, boot from that by holding down the "C" key after the startup chime once the disc is in the drive. Once you're at the welcome screen for the installation, go to the menu at the top and look for Disk Utility. Launch that and then run a disk repair on your drive. Run that 2-3 times. Once that's done and it says everything is OK, run a Repair Permissions task a couple of times. Usually when the permissions have been repaired, you won't see much output as you would doing it the first time around. After you've done that, exit from Disk Utility and then exit from the installation (this should tell you to restart). See if it helps after this.