Clearly this is a problem. It is strange because I have XLC working in Excel 2003 on XP with no problems. The thing that worries me most is when you get the message 'C:\Program Files\XLC\XLC.xll' is not a valid add-in. This could also give the "Run-time error '1004': Add method of AddIns class failed." associated with installation of XLC.dll. I have exhausted all the routes of enquiry and I fear that I might not find a solution for you.

I'll ask an old college friend and programming guru if he has any ideas. I'll get back to you later.