From Drive Software:
The holiday season might be approaching quickly, but most peoples' computer usage doesn't lessen during this busy time of the year. If you want to get into the holiday spirit while still being technologically productive, try Christmas Tree for Desktop.
Christmas Tree for Desktop is a fun, easy to use program to get you in the holiday spirit. You can keep Christmas Tree on top of every window that you work. The Christmas Tree are always on top by default. You can move the Christmas Tree to wherever you want. Plus you can adjust the transparency of the Christmas Tree as well.
The setting can be configured by right-clicking on the Christmas Tree. Does not appear to affect PC performance.