Decorate your Windows 7 desktop with this free Thanksgiving Theme. The wallpaper displays "Happy Thanksgiving" amidst an assortment of cornucopias and autumn leaves, and the system sounds are set to the included Windows Delta scheme (a nice touch). The theme includes fun icons, too: The Recycle Bin is a bowl of cranberries; Computer is a turkey; and so on.
This theme isn't terribly exciting, but it is a nice and unobtrusive way to get into the holiday spirit.
Note: To install this theme, just unzip the download and double-click the themepack file; Windows does the rest.