excel add-in function not persisting
 

I needed the function DEC2HEX so I added the Analysis TookPak (Excel 2003).
Everything works fine- the function works, I save the workbook and close it.
However, the next time I open the workbook Excel throws a #NAME? error for
the DEC2HEX funtion. I check that the add-in is still there and it is. If I
retype the function it works again. What is causing this? How can I make
Excel remember that function?

Thanks for you help,