- Make hidden files visible (View tab, check hidden items)
- DRAG (not delete key) AND DROP everything, except your music files (including thumbs and desktop icons) into the recycle bin
- If you had correct album art before, this should default your music back to original album art
If not:
Look up the album info manually via Windows Media Player - right-click on an album art, select Find album info , search for the correct album, and follow the remaining steps of the wizard.
I know, it is very annoying but I do this steps very often.