It may be that one of the references is missing. Click Alt-F11 and then go under Tools -> References. See if any of the checked references are prefaced with [MISSING] if they are, make a note of what the reference is called, and then uncheck it. If it still gives you the error, and it is some thing like Microsoft Office Objects 10.0, and you are using Microsoft Office 2003 (11) then try and Select Microsoft Office Objects 11.0 instead.
Hope that helps