Well the registry key in WinXP is, to my knowledge(this is where it is on my computer),
My Computer\HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Themes\LastTheme

Then there is a value called Wallpaper. This is the value that changes when you change your desktop background. However, I have been unable to change the background by changing that value.


There should be a way to do it, but I don't know it off the top of my head. I will keep looking though:P

I know you can use batch files and the like to set it so that any time you boot the background is set to a particular setting(or so I have heard). Is this what you want to do?