Various suggestions:
'Single User' mode and 'fsck -fy':
01. Reboot the Mac into '
Single User' mode.
02. Enter ...
fsck -fy
... and press the <return> key.
03. Reboot and use the Mac.
'Install Disc':
01. Insert your 'Install' disc.
02. Reboot the Mac, with the <option> key pressed, selecting the 'Install' disc and '-->' button.
03. Once the Mac reboots, via the 'Install' disc - select the '
Installer, Open Disk Utility' menu item.
04. Click on the Mac hard disk drive icon in the left side list, click on the 'Repair Disk' button (perform at least twice), and finally - click on the 'Repair Disk Permissions' button (perform at least twice).
05. Quit 'Disk Utility'.
06. Quit 'Installer', which should reboot the Mac.
07. Use the Mac.
'In person evaluation':
01. Have someone, in person, resolve the problem.
Then reply with your results.