There are plenty of web sites that can help you resolve now already well known problem in OWA when your browser does not display any pictures in it and hangs with Loading... text.
Use Google to find other resolution if it is not the one described below.
You setup Exchange 2007 into the existing Exchange 2003 environment. You reroute Internet users to use your new Client Access Server (CAS) for OWA. CAS server includes a mechanism that redirects user to the correct Exchange server, being that Exchange 2003 or 2007. It depends on which server the mailbox resides.
In the case that you changed Authentication type on your CAS server such that users don't need to provide domain\username but only username, your Exchange 2003 users could get following screen in their browser...
The resolution to the problem is in the authentication type on your CAS server. Not only that you need to change authentication type on your /Exchange folder but also on your /owa and /Exchweb folders.
I would also recommend that you enter your domain name on existing /Exchange and /Exchweb Exchange 2003 web folders in IIS if you chose to use only username for login.