I've just replaced the windows XP banner on my PC (XP classic logon not the welcome screen.
I used a program called reshack to edit the MSGINA.DLL file.
Open the MSGINA.DLL file and click bitmap from the list. Select the item you want to edit, right click and save the bmp image. Edit it and select replace bmp from the same menu.
Now when the user logs on they see my logo, instead of the XP logo.
Brief instructions but the program is self explanatory.