OK, OK, Windows is not the most popular thing on a site like this. Still, there are some Linux users who want their desktop to look like Windows XP's "Luna" theme, whether it be for pleasing a new and shy Linux convert, or just because they enjoy the irony.
This is a theme for Avant Window Navigator that complements any other Luna-like theme elements. Excellent for keeping non-Linux users guessing whether your using "Windows with a Mac Dock" or a "Mac with a Windows Theme" (I even had one checking the back of my laptop for an Apple logo)!
Also recommended for Windoze fanboys...
Lunacity theme for Metacity: http://www.gnome-look.org/content/show.php/Lunacity?content=62350
Lunacy GTK Theme
GnomeXP Icon Theme