I created an AddIn as described by Excel 2002 Bible and saved it in C:
\Excel\AddIns.

I made several changes and, each time, saved it in C:\Excel\AddIns.

I encountered a problem where it said I could not save an AddIn which
was already known. I solved it by closing Excel then starting again.
This led me to investigate.

After a bit of investigation, I have found that the AddIn was not
stored in C:\Excel\AddIns but in C:\Document and Settings\<User Name
\Application Data\Microsoft\AddIns.

When I look at C:\Excel\AddIns using windows explorer there are no
files there! I've used a command prompt and there are no files there
with hidden or system attributes.

Has Excel somehow redirected the file so that it can be referenced via
C:\Excel\AddIns (this is where I added it from in Tools / AddIns).

Is this an XP feature that I am not aware of?

Confused...