1) Do you have a 75 Ohm cable?
ATI cards a picky about cables sometimes.
2) Does it display text at boot-up?
ATI cards enable TV-out on boot.
3) Do you have the most resent drivers from ATI?
Standard windows drivers have problems with TV-out
4) Have you tried to force TV-out?
Look in the display settings. It is one of the tabs with the ATI logo.
It is one or more of those four problems. Been discussed many times before.