Bad Science
It does sound like a failing hard drive.
Fit another drive; there is a spare bay, underneath the existing drive. I think you need the old style IDE drive, not SATA. Use Carbon Copy Cloner (free download) to clone your old drive to the new one. Set the new drive as your start-up disk in preferences.
Reboot; be lucky.
Bad Science
It does sound like a failing hard drive.
Fit another drive; there is a spare bay, underneath the existing drive. I think you need the old style IDE drive, not SATA. Use Carbon Copy Cloner (free download) to clone your old drive to the new one. Set the new drive as your start-up disk in preferences.
Reboot; be lucky.