Removing Trails of Exchange Installation and Cleaning Up Schema

Hello All Experts,

I am sure there are lot of articles on it but nothing seems to help ms.

We had a failed trial installation of Exchange 2007 in our domain (Windows 2003 SP2). The server name was given as EXCHANGE (unfortunately) and it was also made as an Aditional Domain Controller when Exchange was being installed. The server is now no longer an ADC or part of the domain, totally reomved from the network. I have used NTDSutils to remove the server from the site.

Now we are planning to install Exchange 2007 on a new machine (POSTOFFICE). We went ahead installing Exchange 2007 but got failed while installing Client Access Role (DEL EXCHANGE (old server name) <GUID> was not not found). Also I found when I went into the Exchange Management Console of the newly installed server that I already have some 5 mailboxes (which I later found in AD that some users still have their mail boxes referred with old EXCHANGE server). Now when I tried uinstalling the newly installed server, I am not able to fully uninstall that too :-(   getting the following error:
Hub Transport Role Prerequisites
Failed

Error:
You must be a member of the 'Exchange Organization Administrators' group, or 'Enterprise Admins' group to remove all roles from this computer.

My question is how can I completely remove the trails of old exchange server and clean-up the Active Diretory Schema and install a Fresh Exchange Server.

Thanks in Advance