When adding a trusted domain or migrating computers to a new domain, the login prompt often defaults to the old domain or the name of the workstation. This can cause confusion for users who are often confused about computers by default 
In order to make their lives easier, you can set their default login domain by modifying the computer's registry (reg add available only on XP and 2k3). In the examples below "NEWDOMAINNAME" is the new domain name and "wkstnName" is the recently migrated workstation name.
run from command line..
Running the command locally:
reg add "HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon" /v DefaultDomainName /f /t REG_SZ /d "NEWDOMAINNAME"
Remotely modifying the key:
reg add "\wkstnName\HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon" /v DefaultDomainName /f /t REG_SZ /d "NEWDOMAINNAME"
If you use Win2000, you can modify those values with regedit or download the Windows 2000 SP4 Support Tools which installs reg.exe.
Or you can do it in WSH..
const HKEY_LOCAL_MACHINE = &H80000002
strComputer = "wkstnName"
Set objRReg = GetObject("winmgmts:{impersonationLevel=impersonate}!" & strComputer & "\root\default:StdRegProv")
strKeyPath = "Software\Microsoft\Windows NT\CurrentVersion\Winlogon"
strValueName = "DefaultDomainName"
strValue = "NEWDOMAINNAME"
objRReg.SetStringValue HKEY_LOCAL_MACHINE,strKeyPath,strValueName,strValue
Set objRReg = Nothing
Microsoft also provides a way to do this via Group Policy. Oh, as an added bonus, this also works instantly for Terminal Services. On one of my machines, it would always default to blank or the wrong domain. I tested this script on it and now I get prompted with the most proper domain each time.
keywords: default domain login, trusted domain default login, dual domain login default