link from web page to excel asks for username and password
 

On a pc running IE8 and Excel 2007, when I link from a particular web page to
an Excel workbook, I am prompted for a username and password for the website.
I can click on Cancel, and the workbook then opens OK. On a different pc,
running IE8 and Excel 2003, when I link from the same web page to the same
workbook, I am not asked for a username and password. Why does the Excel 2007
behave differently, and how can I stop this?