How can I replace the image which is shown when using the default "Windows XP" screensaver? What is the registry key of interest? I would like to replace the Windows XP logo with a custom logo.
A quick look round the web seems to imply that the image is hard coded into the screensaver. There's a couple of things you could do though...
1) Use a commercial "make a bouncing screensaver with a logo of my choice" program. Caveat Emptor here though. Screensavers downloaded from the web seem to have a higher than normal incidence of malware.
2) If it is a registry hack, you can download a registry monitoring program (sysinternals has one), fire off the screen saver and see what sections of the registry are read.
3) Use a resource editor to change the image in the default screensaver.
All that's vague, and I apologise for that. Good luck with your search!