I take it you mean teh default wallpaper for the user on his machine..If that is the case you can create a default user profile for win2k and winxp that has the wallpaper you want in it. You can also set folder views and screensaver etc.
If that is what you want all you need to do is create a new user log on as this user set everything the way you want it then log of. Log on as a user with admin rights on the pc right click on my computer icon. Now its diffrent between xp and 2k...in 2k click on user profile tab & in xp you click advanced then settings under user profiles.
Next copy the new user you have created and configured to Default User, just browse and select Default User. (C:\Documents and Settings\Default User) click on Permitted to use change button and set to everyone or you will get problems with user not allowed to change screensaver and other settings, unless thats what you want.
/Kim