In Excel 2003 and earlier, you have only 256 columns, which is column IV.
This is are hard limit in Excel and cannot be increased. In Excel 2007, you
have 16384 columns, which takes you out to column XFD.
Because the file format is Excel 2007 is different than that of earlier
versions, if you open up an existing 2003 workbook in 2007, you are running
in "Compatibility Mode" and are limited to the restrictions of 2003 and
earlier.
--
Cordially,
Chip Pearson
Microsoft MVP - Excel, 10 Years
Pearson Software Consulting
www.cpearson.com
(email on the web site)