Go get the drivers from ATI. There's an option in the display properties (Right click on the desktop, go to Properties -> Settings -> Advanced -> Display tab)
If you don't have the right ATI drivers installed, you won't have the right 'Display' tab which should have a little ATI icon next to it. Inside that tab you can select the 'TV Out' as the primary video.
There's only one video overlay on that card, and you have to assign which is the primary display so it knows where to put it, if you have two displays hooked up, by default it'll do VGA first, with the composite output only seeing a black box.