Excel Add-ins error

all users on my 2000Pro network are having the following problem (running Office XP). when an Excel file is double clicked in Explorer, Excel opens and displays the error "Error loading Add-ins ". Click OK and it opens the file, everything looks fine. The workaround for this problem is to simply open the file by startring Excel first and going through File, Open.
Looking at Tools, Add-ins - only one add-in is checked - Ppcwpbi. Incidentally, the title of the error dialogue box is WPBI. I've unchecked the add-in, closed XL and tried to open from Explorer with the same results. Any ideas?