This and other reasons noted at http://www.winvistatips.com/id3-tags-missing-wmp11-library-t785093.html
I have a PC running Win 7 with WMP 12. I have a number of mp3 files ripped using this program, and everything is fine there. I then copied the files to an older PC that is running XP with WMP 11. The files are in the standard folder structure. The old PC sees the files fine and adds them to the library, but puts them all in one "album" with all tag information missing. I can add one album at a time, use find album info to locate the correct data, and the album will be updated and ok after that. However, that's a reasonably laborious process for a number of albums.
When I look at the files with the tag editor in 11, there don't seem to be any tags set. If I open the files with a binary dump, I can see what looks to be tag data in the header.