Definitely not legal, and debatably ethical -- consider that when torrenting, you're not only downloading but also distributing to untold dozens of other people, most of whom probably don't own the original CD.
Also illegal, but more ethical IMO, and less likely to get you caught (don't forget that one woman who got fined like five million dollars for sharing 24 songs -- would you be able to afford a good enough lawyer to fight the RIAA?), would be to download it off of YouTube if it's up there and convert the resulting MP4 or FLV file with WinFF (this also has the benefit of letting you know exactly what song you're getting beforehand, and ensuring that it pretty much can't possibly be a virus). If you're an audiophile, you'll usually notice a drop in quality, but a true audiophile would already have five factory-sealed vinyls of the song down in their wine cellar and wouldn't have to worry about something like this anyway.