We have found that completing these steps will sometimes fix the problem:

1. Quit all applications
2. Double click the Macintosh HD and go to : Users > your account (picture of a house) > Library > Preferences.
3. Inside this Preferences folder locate the following items and drag them to the Trash:
com.digidesign.protools.plist
DAE prefs (a folder)
Digisetup.OSX
ProTools LE Preferences

4. Empty the Trash
5. Insert your ProTools LE 6.8.1 installer disk and run "Install ProTools LE"
6. After you enter your password and "agree" to the license agreement, change the "Custom Install" button to "Uninstall"
7. Click the Unisntall button and when prompted choose "Clean Uninstall."
8. When uninstall is complete, go to the Macintosh HD > Applications and drag the folder "Digidesign" to the trash.
9. Restart the computer.
10. Emtpy the trash (Finder menu > Empty Trash).
11. Reinstall ProTools LE 6.8.1 from the installer disk. Do not make additional selections in the "custom install screen." Leave only "ProTools LE 6.8.1 selected."
12. When installation is complete, restart the computer and try ProTools again.