KB article: http://support.microsoft.com/kb/314493

SUMMARY

This step-by-step article describes how to change the default logon screen saver...

This step-by-step article describes how to change the default logon screen saver in Microsoft Windows XP. Specifically, the article discusses how to change the type of screen saver that starts, the timeout before the screen saver starts, and whether a screen saver is turned on before the logon process.

When you start Windows, you may be presented with the Windows XP welcome screen, which prompts you click your user name to begin, or you may be presented with a Welcome to Windows dialog box that prompts you to press CTRL+ALT+DEL to log on. By default, if you do not press a key for 10 minutes, the Windows logon screen saver (Logon.scr) starts.