Every time I start Excel it launches a dialog box that says to wait while
Windows configures Macromedia Contribute 3. Then it asks for the original CD.
At that point I cancel and Excel runs normally. After doing this many times
Excel said it had a problem with the Flashpaper add-in and asked if I wanted
to disable it. I said Yes and didn't experience this problem further.

What caused this problem? Can I re-enable Flash Paper?

I do have Contribute 3 installed on my computer and it works normally.