If you're talking about editing only the metadata (i.e., not the audio itself), try
MP3tag. The same download can either do a permanently install or a portable install, and even the latest version still works with Win7.
If you want to also edit the audio (volume levels, equalization, et al) along with the metadata, use Audacity. But if all you're doing is editing the metadata, I'd recommend MP3tag instead because I think Audacity might unnecessarily re-encode the audio every time you save. Also, Audacity is only good for one MP3 at a time, whereas MP3tag can batch edit a whole bunch of MP3s, which is handy if you're changing a whole album's worth of MP3s to show consistent metadata.