Works & Doesn't Work - Excel

I've developed a fairly simple Excel Workbook (2 Sheets) and each sheet has some buttons which will run some code. This works fine on my PC where I developed it. But when I try to run it on the user's PC, it doesn't work. Some of the code is simply ignored.
My Excell is: MS Office Excel 2003 (11.6355.6360) SP1
My Windows is: Windows XP
User's Excell is: MS Office Excel 2003 (11.6355.6408) SP1
His Windows is: Windows 2000
I don't see that the Windows difference would be the problem. The code I have is nothing tricky or special. It is code I've used and still works on my Laptop Windows95.
Any idea what might be the problem?
Many thanks,
Michael