is there a video adapter that will....?

I'd like to feed a channel on a private cable television
network using a windows xpsp2 computer. Composite NTSC
output is required, using RCA connectors.

I'd like to run a software movie player, such as MPC (Media
Player Classic), and have the NTSC composite output contain
only the picture part of MPC. No other part of the 1280x1024
computer display would output to the NTSC composite connector.

I hope to update the MPC play list while MPC is playing a
movie that is being broadcast to the cable channel. The TV
viewer would be unaware of such play-list activity.

Is there any video adapter and driver that will output a
specified fraction of a computer display?

--David