Automation error with Common Dialog Control

I have a VBA application in MS Word which includes a Common Dialog Control on one of the forms. On some machines, An 'Automation error' occurs when Word tries to load the form in question.
I have managed to track the problem down to the Common Dialog Control, because if I remove this from the form, it loads and the application runs (but for the vital load and save facilities).
The relevant files for this control appear to be Commdlg.dll and Comdlg32.ocx. So I have replaced these on a machine which has the problem, using the files taken from my machine (which does not have the problem).
However, this hasn't helped at all.

Furthermore, if I try to view the properties of this offending Common Dialog Control it comes up with a non-descript System error.

I've tried re-installing Word ('97 & 2000) and have run out of ideas completely.

If anyone has any ideas, please share them!