This won't work with Longhorn but in XP you can change the LogonUI without software.

Firstly Enter Regedit, then go into 
'HKEY_LOCAL_MACHINE>Software>Microsoft>Windows NT>Current Version>Winlogon.

Under here change the 'UIHost' to whatever location your new logon Screen is kept, for example 'C:\WINDOWS\Resources\Logon\glass palm.exe' instead of 'Logonui'.

Remember to include the .exe extension and DO NOT RESTART OR LOG OFF, instead Press Winkey+L to test it.