In many cases trashing ProTools Preferences and Digidesign Databases fixes this problem:


Deleting Digidesign Databases:
1. On the root level of every mounted hard drive, go into the Digidesign Databases folder and delete the "Volume.ddb" and "WaveCache" files.
2. Go to: My Computer > C: > Program Files > Digidesign > Pro Tools > Databases > Volumes. Delete all of the files in this folder (they will have very odd names).

Trashing Preferences:
1. Go to: My Computer > C: > Documents and Settings > (your user name) > Application Data* > Digidesign. Delete the file entitled "ProTools...v6.x Preferences.PTP" (this may be either ProTools LE or TDM, and any version, such as 6.4 or 6.9).
*If the Application Data folder is not there, go to: Tools (in the menus at the top) > Folder Options > View, and select "Show Hidden Files and Folders."
2. Go to: My Computer > C: > Program Files > Common Files > Digidesign > DAE. Delete the entire "DAE Prefs folder."

Empty the Recycle Bin after trashing preferences and/or databases.