I don't believe it's AVG either, and I'm certainly not going to turn off my virus protection to thwart what acts like a virus! In any case, none of the files are disappearing; they are just being removed from within the Itunes Library.
It is actually easy to "solve" the problem. I just click on "Add File to Library" and add all my music back in. The problem is that I've done this at least 20 times by now. I'm stubborn, but not stubborn enough that I'm going to put up with this much longer.
If AVG were the culprit, I'd expect the files to be removed from the Library after an AVG scan or an AVG update. That isn't the case. The files are being removed on a regular basis, so I assume some update is creating the problem; I just can't figure out the timing to get any idea of whose update might be the cause. Likeliest candidate is the Itunes updates, though, so I've disabled those for now.
I've also followed the advice from this thread (http://discussions.apple.com/thread/3678846?start=0&tstart=0) to make the Itunes Library files an exception in AVG's Resident Shield, in case something in there is acting like a virus. I haven't gotten any warnings from AVG, but I'm still not too happy about that work-around because the problem could BE a virus....
Anyway, Itunes still contained the Library when I turned on the computer this morning. I expect my music won't go missing again until later today. In every previous disappearance, all that remains are the few purchases I've made from the Itunes Cloud.
Maybe Apple is trying to tell me I'm not worth it as a customer. I'm sure getting that message.