I'm trying to hook up my TV to my computer as a gaming/multimedia center but I'm having troubles setting it up.

I have a custom built machine running Windows 7 RC. It has an ATI Radeon 4800 video card with 2 dvi output and 1 S-video output. I have an s-video to composite adapter that connects to my tv. (It's an old TV with only Cable, composite, and s-video connections).

I can switch the desktop to my TV without a problem, but I can't duplicate or extend my desktop onto it.

I've installed the latest drivers and Catalyst Control Center, but it won't let it work any more readily than Windows would.

Any suggestions? Would using an s-video cable instead of the adapter change anything? (The only reason I use the adapter is because it came with the graphics card)

(Edit)
I installed the latest drivers and I can now duplicate the screen (show on one monitor and on the TV), but I still can't extend the desktop.