OK, OK, you’re right – if I’m going to cover customizing the loginwindow of Mac OS X then the least I can do is cover Windows as well. Because the registry is basically a bigger, more monolithic version of the defaults domains, we’re going to do pretty much the same stuff at the login screen for Windows. First, open the registry editor (regedit) and browse to HKEY USERS .DEFAULTControl PanelDesktop
Then edit the value for wallpaper, typing in the path to the image you’d like to use. To set it as tiled add TileWallPaper and set it to the integer 1 or to stretch add WallPaperStyle and set it to the integer 2.
The acceptable use hint at the login screen is a little different. Open the policy editor (Control Panel, System & Maintenance, Administrative Tools, Local Security Policy). Open up your Local Policies and click on Security Options. Then find Interactive Logon: Message Text For Users Attempting To Log On and right-click on it, choosing Properties from the contextual menu. Then enter the message you want displayed and click on the OK button.
You can also edit the title bar of that box (although that seems a little extra much) using the Interactive Logon: Message Title For Users Attempting To Log On similarly.
Alternatively (if you’re looking at a one-off customization) you can use Logon Studio from StarDock and do all of this from a GUI.