This article tells how to unlock iTunes songs so that they can be played anywhere by the person who bought the tune (your personal information stays embedded in the tune):
How to play purchased music on other systems – Engadget – www.engadget.com.
The instructions are based on using a utility called HYMM (Hear Your Music Anywhere…. which looks like HYMA to me.)
UPDATE Jan 3, 2008: iTunes gets updated so often, Hymm has not been able to keep up, and no longer works with newer versions of iTunes.
An alternative is to “burn” your iTunes songs to a virtual CD (so you don’t have to waste a real CD) and then reimport them. Here is an article on that approach:
How to Unlock iTunes Songs Without Burning a CD
This didn’t work for me. For Windows you have to use DOS commands which means you have to type in some ridiculously long path name. But even once I got that right it told me I didn’t have DRM on those songs and I do. So I’m not sure what’s going on.