Missing References in VBA

Sometime when I run vba code within excel I get the following error message "Can not find project or library". When I goto the tools menu and choose references I see that one of the referneces is labeled "Missing". I uncheck the box and run the VBA code again and runs with no error messages. It seems that this problem occurs randomly. My questions are :

1. Why are the references missing?
2. Is there a way the find missing references and remove them programaticaly.

Thanks

If these references are there, someone put them there for a reason, they may be used by another part of the application. You should verify what these references are and register them on the PC in question. References are usually DLL and/or OCX.