FQDN logon failure

I am in the process of ungrading exchange 5.5 SP4 to exchange 2000. During the forestprep stage I was asked for the 5.5 service account I entered it which correct current password and got the message user account has to be valid. Well it is in use so it is. I have logon failure auditing enabled and this indicated bad username or password. Again user name and password are in current use (on exchange 5.5 services)so they are alright. Then I tried to logon as that userid
userid/password/domain works but
userid@domain\ notice slash fails and this is the form the forestprep requires anyideas???????????