There is only one file type that uses the .LOGONXP extension:
LogonStudio Windows XP Logon Screen
Custom logon screen format used by LogonStudio, a program for customizing the Windows logon screen; may include a background image, as well as custom lettering and buttons that can be applied to the Windows XP logon screen.
NOTE: LogonStudio logon screens for Windows Vista use the .LOGONVISTA extension.
Updated: November 7, 2008