The installer is seeing a non-stock install of Pro Tools 5.1 This can be a alternate DAE or Mixer plug-in, Pro Tools 5.1cs3 or any other number of things. The easiest way to deal with this is to perform a clean install after removing all currently installed Pro Tools Software:1) Trash the Digidesign Folder2) Trash the DAE folder (You may want to move the Plug-ins folder and Plug-ins settings folder to the desktop so that after you reinstall and update you can just drop in those Plug-ins and settings back to the new folders created by the install and manually run the plug-ins update from the installer)3) System folder, trash all Digidesign Extensions and Control panel.4) System folder, trash the Pro Tools preferences, DigiSetup and DAE prefs folder.5) Reinstall Pro Tools 5.16) Run the 5.1.1 updater