Windows XP – Unable to access a FQDN for a Roaming user in IE6

A Windows-XP user with a roaming profile cannot access an internal website using a FQDN, eg. http://xyz.com. on any PC. It opens a logon page, when login is automatic. However, this user can access the website using just http://xyz. This does not affect other users on that PC or other PCs. There is no proxy server and systems are connected using Windows 2003 servers and IIS wbeserver based application