This was annoying. iTunes 7.7 quit playing tracks--any tracks, not just DRM-infested ones--on my Vista installation recently. There was no sound after hitting play and the playhead would not progress. I ran down the suggestions from Apple about this problem to no avail.
After googling, I discovered this post where the author had trouble with video playback after attaching a ReadyBoost-configured USB drive. Sure enough, I removed my ReadyBoost drive and rebooted, and voila--iTunes is again playing songs.