In one case (PT 7.0 LE on a Digi 002), this was solved by manually deleting the Digidesign drivers and cleaning the Digidesign items in the Windows registry, then reinstalling Pro Tools.

To delete Digidesign drivers:
Go to c:\windows\system32 directory
Go to menu item View->Choose Details
Select "Company"
At top (column headers), click on the new "Company" header (this will sort all files in ../system32 by company. All Digidesign files will be now grouped
Select the files do NOT select the "Digi International files, those are NOT Digidesign files!!!
Delete the Digidesign files.
Go to c:\windows\system32\drivers and locate and delete "dalwdm.sys" and "digifilter.sys"