Be sure your TV settings are correct (located in ATI's control panel). Settings should be tuned to S-Video and NTSC.
Also, have you tried plugging into another TV? I used to have the problem past with a certain GeForce3 card under Windows 2000 - apparently XP fixed the problem, but after re-installing win2k on the machine, the problem occured again.
If supplied, try using a composite (RCA) cable and see if the same results.
As a first resort, I suggest using TV Tool
(helped with my win2k issue)