TuneUp Media

The product being sold at this website claims to clean up all of the information and album artwork in your iTunes library.

I doubt I'd ever pay for something like that, as I'm pretty obsessive with doing these things myself for my own collection, but I'd recommend it to everyone else whose mangled collections I have to witness. There's a trial version you should check out first if you're thinking about it. TuneUp supports both Mac and Windows.

There may me free alternatives, all listed at this website. I can't vouch for any of em'.