Given what you wrote, I'm guessing that the application was developed with a reference set to the Excel 10.0 Object Library (aka Excel XP). The object libraries are (for the most part **) forward compatible but not backward compatible.

If this is the case, go through your code and change all your Excel object references from early binding to late binding. You'll get a slight performance penalty, but for most operations this isn't even noticable.


** Many developers use forward Excel object library compatibility without a hitch, but Microsoft has stated in ther migration whitepapers that early binding is preferred because there are a few forward compatibility issues.