On my recently purchased 140 CD's Adobe 5.1 installed as part of (what I call) the hook software on the computer. This is even though I have Adobe 7.0+ installed as well.

I think that the older MB CD's don't work very well with the later versions of Adobe, which my CD uses to display the actual topics contained on the CD. I do know that when I changed the association for PDF files back to Adobe 7.0+ from the 5.1 setting (which was put in place when I installed the CD), the MB CD wouldn't work correctly. Restoring the association (done via Windows Explorer - select Tools, Folder Options, File Types and find PDF in the list - then point it to C:\Program Files\Adobe\... don't remember the rest of the path, but look for Acrobat 5.1 or something similar, then Reader and inside that Acrord32.exe.)