According to Microsoft
, Office 2003 is compatible with Windows 7. I wouldn't recommend it though. Is it financially possible for you to upgrade to 2007? If not, have you tried removing and reinstalling 2003 yet? Doing so should not mess with any data or Office application settings. That data is not modified (or removed) by the Office installer.
If that fails, does AT&T have any documentation on their site about configuring Outlook to access their mail servers? Most ISP's do, even if their support staff tell you to use their own webmail service instead.