Exchange 2003/5.5 Problem

I have an Exchange 5.5 site and I am currently in the process of migrating to
2003. I have my first Exchange 2003 server setup coexisting with Exchange
5.5 in the Exchange 5.5 Site. I am trying to setup an additional Exchange
2003 Server (Cluster) in the 5.5 Site. When I start the setup and after it
asks me if I agree with the licensing agreement, it asks me to supply Service
account information for my 5.5 Exchange site, but the userame and domain are
grey’d out and only the password box is available. If I select next then an
error dialog pops up telling me that I have to select a valid account.
Anyone ever had this problem before?