Problem with alpha channel

Quite new to blender, trying my hand a 2d scene like the opening titles in Kung fu Panda using PNG files for the separate layers. When I come to render the scene the edges are all pixelated and with white lines, any suggestions to where i’m going wrong would be greatly appreciated.

http://www.flickr.com/photos/[email protected]/3042823063/sizes/l/

the link show the lines around each layer