Okay, I uploaded the Original Windows XP Luna Theme.
You can find it here:
Download that, and extract the files to %SYSTEMROOT%\Resources\Themes\
That should put it in the correct directory. I'm not sure which dir Folder Windows will be for your computer. Since you Upgraded from 2000, it's probably c:\WINNT\Resources\Themes, but it could be c:\Windows\Resources\Themes
But, thats why I typed %SYSTEMROOT%\Resources\Themes\ because if you enter that into the prompt for where to extract, the "%Systemroot%" will tell it to use whatever directory you have for windows.
Anyway, when you extract that you should just be able to either click on the Luna.Theme file, or go to the display properties and change it there.
If you have any problems with it working, only other thing I could suggest would be to download the UXTheme.dll patch (http://www.neowin.net/forum/index.php?showtopic=140707
), which will make it so you can use other downloaded themes besides Microsofts, and download a different theme off http://www.themexp.org
Yay for making posts more complicated then they need to be.