Hi there,
to do the 'Disk Repair' you need to boot from the Install CD or DVD. Use the latest one that you have ie if your Mac came with 10.2 Install disks, but you have since upgraded to Tiger 10.4 then you must use the 10.4 Install disk...
Pop the disk in, restart hold down the c key and then when the Installer comes up...don't install ! But go to the menu bar and select Disk Utility...run Disk Repair...then 'Repair Permissions' then quit Disk Utilities...then quit the Installer...then Reboot.
regards
Ric