Do not disable the following COM add-ins (enter ProgID):

By default, any existing Excel COM add-in is disabled when loading a secured workbook.

However, some useful add-ins like Microsoft PowerPivot may be required by your workbooks. In that situation, enter all progIDs of the different add-ins you want to keep in the field separated by semi colons:

Company.Addin1;Company2.Addin2.File;…

For instance, for Microsoft PowerPivot, the ProgID are: Microsoft.AnalysisServices.Modeler.FieldList and PowerPivotExcelClientAddIn.NativeEntry.1

Learn more at http://support.microsoft.com/kb/291392/en-us