I'm sorry, I'm not answering your current problem here as I don't use iTunes for music any more. But for future reference, I highly recommend you start buying your music from DRM-free sources like Amazon's mp3 store. The biggest reasons being that you actually have more ownership over your music than iTunes DRM music for the same price, and you can make ringtones from them for free using Apple's Garageband software or other free music software if you don't have iLife '08. Amazon is a very good alternative to iTunes because the music is DRM-free, Amazon also has one-click shopping, and their downloader will automatically import the music you buy into iTunes. With Amazon's service available, there's honestly no reason to ever buy DRM music from iTunes again.
But as for your immediate problem, if the music you bought happens to be DRM-free iTunes Plus music (which I'm guessing it's not) you can still make free ringtones with it which is perfectly legal and easy to do, and save yourself some money.