Resolution is too high, even though SD TVs can't really do 800x600 that does seem to work over s-vid. That is also what the splash screen for XP is (at least I think, otherwise its 640x480). When it kicks to the desktop or login screen the resolution is now too high to go to the tv over s-vid, you'll have to drop it back down at least on that output depending on your card and drivers.
Check out the advanced properties where you change your screen resolution.
/edit: even if you are going to an hdtv, you are still going over s-vid, which isn't going to like anything higher than 800x600