You may need to open the Catylst Control Center in order to do this. Not sure how it works with ATi since i run nVidia, but i know that when i right click anywhere on my desktop there is a option there for the nVidia control panel. In there is the option to setup multiple desktops. So you can try that. can also try through the windows display settings as well.
Right click on the desktop, select Personalize, in there click on Display on the lower left hand side under See Also. There on the left side again select the option for Change Display Settings. That will give you the option to expand your desktop to the TV.
To give Rep click on the check mark icon under the username. It should be located right near the yellow triangle with the ! in it.