I have a RCA 25" Tv with S-Video and read a while back that a few people had the problem of it always resetting to Video Input instead of S-Video, including me. I found out how to fix this. Go into the menu, then go to channel, then go to "Video Input Source" FIRST. After you set this you can then set one of your buttons in "Auto Tuning" to Video Input and it will always be set on S-Video when you turn the TV back on.
Now if only there's a way to have the TV automatically turn on when power is on :/