Without a screenshot it is a bit difficult, however, let me take a stab...
The XP look on the login screen comes from (I believe) the performance options (right-click My Computer -> properties -> Performance -> Settings -> Visual Effects) If you uncheck "Use visual styles on windows and buttons" you should be able to have the "XP look" go away from the log-in screen. Unfortunately, this also holds true for the XP start bar and title bars, etc. Try it, see if it gives you what you want. Looks like Win2k.
The background color at the login prompt can only be changed by a registry edit. Use an RGB color value and edit the key
HKEY_USERS\.DEFAULT\Control Panel\Colors\Background
to equal some RGB value.
"Background"="0 0 0", for instance, will give you a pure black background. "Background"="255 255 255" will give you white.
If you want to change the default background at the login prompt to be a bitmap image, edit (or add) the registry key:
HKEY_USERS\.DEFAULT\Control Panel\Desktop\Wallpaper
to reflect the bmp file you want to use, e.g.:
"Wallpaper"="C:\WINNT\Web\Wallpaper\whatever.bmp"
As ever, when editing the registry, always know what you are doing before you do it. You broke it, you bought it.