[TheBang]

Some tracks sold by iTunes are iTunes Plus, which lack any DRM controls, so you can transcode those. For those that are "locked", put them into a playlist, and then burn that playlist to an audio CD (click the "Burn Disc" button in the bottom right corner). Once they are on the audio CD, they are standard, unprotected CD Digital Audio, and you can rip them to any format you wish.

For those of you having trouble just playing them in iTunes, what exactly is the error you're getting? You shouldn't have trouble playing them unless you've forgotten your Apple ID and password that you purchased the tracks with. Or if you exceeded 5 computers authorized with your Apple ID. If that's the case, log into your iTunes account, by going to the iTunes Store, and pressing "Log In" or your Apple ID in the upper-right corner of the Store. On Your Account screen, it will show how many computers are authorized. If you've reached the limit (5), a button will be available to "Deauthorize All" so you can start over.