Freeware from the site that gave us the brilliant Atomic alarm clock some time back.
Animated Christmas Tree for Desktop - 2009
Get free Christmas tree for your desktop. Decorate your desktop Christmas tree.
Add a fresh touch to your desktop with Christmas tree.
It can optionally stay on top and you can adjust transparency - a mouseover hint reveals how many days are left till Christmas.
Link: http://www.drive-software.com/desktopchristmastree.html