When you logon do you have the Classic logon screen and press Alt+Ctrl+Del. If you do what is in the bottom box. Is it the name of the workstation or is it a Domain name. If it is a Domain name change it to the workstation.
Extract
Windows 2000 and Windows XP handle duplicate down-level account names as well but in a slightly more intuitive manner. A suffix is placed on the username of the profile that is either the name of the domain, if the user account is a domain account, or the name of the computer, if the user account is a local user account. If, by chance, another user with the same name from the same domain or computer logs onto the machine, Windows 2000 or Windows XP adds a .000 suffix to the domain or computer name. If the action happens again, it then starts incrementing the .000 as well.
http://support.microsoft.com/kb/269378