and just how big are your codecs? does your system hang a lot with it being used? nvidia codec has proved to be more stable on my twinhan back home compared to other codecs (even if i used an ATI video card to boot).
now as for your child protection on mytheatre, first try to set the lock code via Actions>lock Channels, try to right click on your channel list, and select channel properties. if you see "lock" then you are in the "parent mode" and you should be able to lock your unwanted channels. if you cannot see the "lock" box, you are in "child" mode. to switch between parent and child mode, just view any channels, and enter your six-digit numeric password (it HAS to be numbers, use the numbers on top of the keyboard, not the numeric keypad, just TYPE your numbers like you are changing the channels). mytheatre then should inform you that it has switched mode. the locked channels will not be visible to the child mode (mytheatre boots/starts in child mode).
the password lock can be found under settings.ini under section [MyTheatre_Common] and then remove options=xxxxx string. good luck.