I've been using Windows 7 for about a month now, and I initially enjoyed WMP until I started having this problem. It first occurred after I'd been using for a week or two. Songs I had just added (through the player watching my "My Music" folder) would seem to disappear at random. Not from my hard drive altogether, just the library. Usually, it was some or most of an album I'd just added, but never the entire thing. The first few times this happened, I had to re-ad the music by going to Tools > Advanced > Restore deleted library items. That worked, but unfortunately, any files had lost their formatting I had changed (i.e. capitalization, names, etc.). It was just as though I had added 2 tracks yesterday, and then the other 10 later today.
Then I started to notice that this seemed to happen most when I was editing some aspect of a song. Whether I was using the "Find Album Info" feature, or changing the album title so it was more consistent with all the other data, or even changing the year of an album from Unknown to 2009, it would wind up removing some of the items. I initially thought it may have to do with that I had it set to rename files to be consistent with the rip music settings (i.e. change "01-rick-astley-never-gonna-give-you-up.mp3" to "01 Never Gonna Give You Up.mp3," but two things make me wonder if this is it at all: 1) it wouldn't remove every file, just most of them, and 2) it even happened when I wasn't editing anything to do with the file's name (i.e. year). I even had it once remove a song I was in the middle of listening to.
Anyone have any idea why it's doing this to me? Corrupt library (which, incidentally, I had that problem on my old computer with XP MCE and WMP11, although there it ceased to add any music, despite numerous attempts to start over). Maybe WMP12 has become sentient, and just doesn't like certain songs?