Got the Leopard OS X.5 original DVD?
If so boot from that, hold down 'C' after the chime, leave Installer and go to Utilities > Disk utility > Repair Disk. You cannot repair a disk you are booted from. See what message comes up. If any in red as Nick suggests the drive may have/be failing.
If you need a new drive, pop it in, boot from the DVD again, run installer after formatting the drive Mac OS Extended (Journaled) and then withg the external connected use Migration Assistant in utilities to cross evetrything over from the Time Machine backup.
Keep us posted?