Usually for s-video, you right-click on the desktop, select properties, then select the settings tab, and click the advanced button, and there should be some options there.
For me, I use s-video to my TV off a nvidia geforce 4 ti4200 video card, and I ended up installing a piece of software from the nividia site, so that whenever i right-click on the desktop, i have a new option "NVIDIA Analog Display", and in there i can configure my s-video settings, im not sure if this will be the same for you. Then fiddle with the video settings on the projection tv.
Sorry im not much help... I think if you posted what video card you had it would help the more knowledgeabe members on the forum help you. I'm just a noob myself and you probably shouldn't listen to be, but listen to the forum staff