The error is coming from a macro. If it appears when you open a document
(not when you create a new document?) look for a macro called AutoOpen or a
macro called Main in a module called AutoOpen. The quick fix is simply to
delete it; but you should ask why it's there in the first place -- it's not
part of Word itself: it's been added, either explicitly by you or another
user, or indirectly when you loaded some add-in or other code.