OK, my understanding of your question was right then. You are actually complaining about ATI's software. Can you find the A/V input under Media Center?
With ATI Multimedia Center, you can right click the video screen, and you should see "Input Connector" submenu, where you can select "Analog TV Tuner", "S-Video", and "Composite".
With Media Center TV, you run Options->Scan to scan for all available channels. This should give you all available analog channels (if you have an antenna or cable connected) AND two special "channels" - "Video Composite" and "Video SVideo".
If you can not get these, then there must be some problem with you hardware/driver installation.