Replication Fails On Login

I am trying to correctly configure a very small replication - one table - between two test machines. Publisher is Windows server 2000. Subscriber is Windows server 2003. Both machines run SQL Server 2005 Enterprise Edition, SP 2 has been applied. I have created an alias from Publisher to Subscriber. In SSMS, I am able to connect to both pub and sub databases (sub is Northwind, by the way).

I was receiving Error 18456, State 11; now I'm reverting back to state 5, which indicates an invalid userid.

HOW do I set appropriate userid for replication? I have established identical id and password on both machines - the only difference between the IDs is the machine name. However, the ID I wish to use is NOT the ID that I use to log onto the publisher computer. This userid is inserted because I'm using Windows Authentication, and I cannot find any means to override it. So I set up the same ID on the publisher, but it doesn't matter what ID I use. I only have TCP/IP turned on because these machines are on different networks, and neither machine is joined to a domain. The alias uses the IP address (but we know that works because I have connectivity under SSMS).

Where can I turn and how do I get this done? I've been struggling with this for 3 weeks, and I'm still not convinced I'm asking the right question.