I couldn't quite figure out why Windows Movie Maker was suddenly chopping off the top half of a video I was making..
I didn't investigate the problem very closely because I finished the project using Sony Vegas instead. However, it turns out that I am not alone.. an InfoWorld post also indicates that there are problems with Adobe Premiere Pro, Techsmith Camtasia Studio, Serif MoviePlus X6 plus some games due to the MS13-057 update pushed out a week ago.
If you are experiencing critical problems with missing video, then the only thing to do seems to be to uninstall the Windows Media Player patch listed as KB2803821 or KB2834904. If this isn't causing a problem then you may as well keep the patch in place to protect your system. I would expect another patch to be re-issued soon.