Help !!!!

I have written an Excel spreadsheet which has been distributed to our clients, but 3 out of about 50 are giving an error 'compile error in hidden module: this workbook'. I can't reproduce the error on any of my test PC's so until I visit the client and unprotect the macro I can't say what it stoping it running. But before I do that I want to get some ideas what could cause it on odd PC's. I wonder if it may be that there is a file (dll, ocx, tlb, etc) missing on these PC's.

So, my question is: When References are selected in MSVB in Excel (Tools, references) are they attached to the Excel file, if not then would I have to make sure they are on the clients PC?

If I understand it right, the required files need to be registered. Is there a way of listing registered files on each PC?

Any helpful advice would be much appriciated as I am new to this (as if you can't tell !!)