Can't connect to SQL Server via ODBC

I have created a user on a Win2K SBS system using active directory. I have added this user to SQL Server as well and given him access permission to a database. However, when this user is logged in to a Windows 2000 pro workstation and tries to use an application that uses a drop down list based on an ODBC connection to this database he is presented with an SQL Server log in dialog. (ODBC DSN uses Windows authentication.) His ID and password are rejected and he must log in with some other ID. (Pretty annoying when using a drop down!) Other ID's that are set up like his do not experience this problem. Can anyone say what the correct SQL Server permission setup must be? I swear this should not be happening. The error is "SQL Server Error: 18456 Login failed for ". Any clues appreciated. More detail upon request.