Galaxy Zoo is a project which uses crowdsourcing to help classify images of galaxies. It's also a great way to find interesting starscapes. Here's how to create your own unique starscape background image using Galaxy Zoo.
Step 1: Sign Up and Start Working
Create an account on Galaxy Zoo, if you don't already have one. Start classifying galaxies. Most of them are fuzzy blobs but every now and then, you'll find a good one.
Step 2: Locate the Object
When you find an interesting object, open up the SkyServer Object Explorer by clicking on the object ID on the upper right.
Step 3: Get a Better Look
Click on Finding Chart, on the left, to open the Finding Chart tool.
Step 4: Adjust and Save the Image
The width and height parameters default to 512. Type in the actual width and height of your screen. In this case, it's 1024 x 768.
Adjust the zoom until you have the starscape you want. You can also play with the ra and dec parameters to center the image better.
Once you have the image you want, save it as your background image. (Firefox: right-click->"Set as desktop background", IE: right-click->"Set as background")
Step 5: Admire Your New Desktop
You should now have your own unique starscape background, probably containing galaxies no other person has viewed before.