cannot open file from macro

I am using a vb form to create an interface for importing and charting data from external files.
While running the form from the vb editor everything works fine.
To make my form appear for the users of my spreadsheet I placed Form.show in the Workbook_Open subroutine. When launching my form this way I get Runtime error 52 "Bad file name or number". Usung the debugger I checked the file name and it was correct, I also changed my code to use FreeFile to select the file number for me.
Any help would be greatly appreciated.