Use iTunes to burn the purchased tracks to an audio CD and the re-rip them to MP3 format.
That'll effectively remove the DRM protection form them, however there's some side effects of this process.
1. It's a violation of your End User agreement with Apple by doing this.
While they allow burning of purchased tracks to audio CD... they do explicity prohibit reimporting those tracks for the purpose of DRM circumvention.
They'll never know you did it, and nothing bad will ever happen to you... but I feel it's worth mentioning.
2. You may lose a little audio quality in the process, since you're converting from one lossy format to another.