Outlook asking users to login to exchange and security certificate

We have a mixture of exchange 2003 and 2007 server. Our goal is to eventually only have 1 server running the Hub/CAS another one running just the mailbox while the final one would be running the edge. So far we have done that, however we want to decommission the 2003 server which use to run the whole environment and also one that was initially running exchange 2007 and all the roles. So far the decommission is running but we have came across number of problems.

1. Outlook is now asking users to provide username/password for the 2003 server which is incidentally not running any roles what so ever now, the login dialog box appears every time they start their outlook. What could possibly be causing this and how can we stop this.

2. Outlook 2007 is displaying the security Alert about the server certificate, my question is is there a way we can stop this security alert popping up without buying a security certificate from one of the known companies?

Thanks in advance for your help.