How to Change the Windows XP Startup Background
- 1). Open the Windows Registry Editor by clicking on the "Start" button, clicking on the "Run" button, typing "regedit.exe" into the Run window's "Open" box and then hitting the "Enter" key.
- 2). Use the plus symbols in the left pane of the Registry Editor to navigate to the "HKEY_USERS\.DEFAULT\CONTROL PANEL\DESKTOP\" subkey.
- 3). Double-click on the "wallpaper" registry key and type in the path to the new image you want as the background (for example "C:\pictures\newbg.bmp").
- 4). Click on the "OK" button, restart your computer and log back in. Your startup background is now changed to the image you specified.
Source...