I decorated my computer for Christmas today. Here is my desktop background:
I use a MacBook Pro, and the colored lights around the edges of the screen are made by an application called MacLampsX, a freeware app that you can get here. It makes little bulbs, you can download or even make new bulbs yourself, you can make them blink in different orders, and it has a built-in music player and screensaver so you can have your own selections of Christmas music play when you want them to. Not bad for a freeware app!
The cute snowmen around the tree background was another freebie I found online. You would not believe how much Christmas stuff there is for free online, for your computer background pictures, anyway. I found a couple of pages of Christmas backgrounds HERE and also HERE (the snowman picture is from the first one), but just do a quick Google for Christmas desktop backgrounds and you will find hundreds of pages of pictures to download. I have another one with Christmas ornaments & lights, and another with a cute little grey kitten in a Santa hat, and another one with funny sheep, all of which I will be rotating throughout the season.
Have a happy decorating season!