Change Logo when you lock your computer WIN XP

I'm trying to change the logo of the small dialogue box when i press WINDOWS + L key to lock my computer. I've tried to hack into a lot of different logon files in c:\windows\system32, but can't find this one image. I would also be interested in changing the login screen. Not the one with fast user switching, this is a different screen after you join the computer to a domain.
Any pointers to the file or images would be great.
P.S. Even changing the CTRL ALT DEL screen would be nice
Thanks