Avast!!
I have 64 bit Windows 7 Ultimate and since 2011-01-22 I had problem playing Blu-Ray movies.
PowerDVD ultra 10 would not play disc in my LG blu-ray player and would not play my ISO on Slysoft Virtual Clone Drive.
Tried different movies. No problems before (6 months).
Message was I was using player for the first time which was wrong and promted me to chose a region even though I allready had and there was 0 (zero) region choises left.
I reinstalled the OS but the problem persisted.
I installed Daemon tools ISO lite but the problem persisted.
I blamed Cyberlink (and Hollywood DRM mafia) and bought Corell WinDVD.
Today I read that this problem have been around from time to time since early 2010.
Most had Avast and 64 OS so I removed Avast and installed Microsoft Security Essential and now everything works.