Go into your control panel and check your power management settings. Go into Control Panel; System; Performance Information and Tools; Adjust Power settings; Choose when to turn off -- check and make sure you have to never or like an hour
Does this ONLY happen when you are watching tv? do you have anything on your tv or BlueRay player that has DLNA enabled that lets you browse your computer's pictures and such on your tv?
Is it turning on from shutdown or from sleep mode?
If it's only from sleep then right click your network adapter in device manager > properties> power management and make sure that "only allow a magic packet to wake up the computer" is checked.