Dual screen problems with Radeon 3600 series

I recently upgraded to vista ultimate from xp pro,on my xp setup I had a dvi to hdmi cable going into my tv which i use to watch movies etc through the pc. I had it set up as an extended desktop so I could still use the normal monitor while the movie played on the tv.
I have exactly the same setup now I have vista ultimate installed, but if i drag vlc or windows media over to the extended desktop and try to full screen it, it jumps back to the pc monitor and fullscreens on that one. Any ideas how i can get it to fullscreen on my tv?
Thanks