Microsoft publishes creepy Halloween theme pack for Windows 7
The wallpaper images vary in quality -- some of them look like amateur attempts with point-and-click cameras -- but with 15 images to select from, there should be more than enough to sate your ghoulish urges.
This is also the first theme I've used with customized sounds. All of your major Windows sounds are replaced with spine-tingling horror film-esque counterparts. A blast from a church organ accompanies the connecting and disconnecting of USB devices, while some kind of violin-and-ghost combo will give you the willies when you receive a new email.
I just wish someone had told me about the new sound set. That's the last time I keep my volume turned up, I assure you...
[Direct link to the theme pack]