u didnt get my question or never heard of it?The question isnt clear
If you are talking about Changing the Screen Saver which comes at the login screen,
Then Go here
*support.microsoft.com/kb/314493
i use this boot editor
userxp.belchfire.net/indexbe.htm
really easy and portable
and if you are looking for theory about how logon screen editors work
i dont know
so now what are you looking for