Ok... is this correct?
You're using integrated video right now for the monitor
You have an AGP mx440 in the computer too with the projector plugged into it via s-video
If that's the case, you don't need to use the onboard video anymore. Plug the monitor into the vga connection of the mx440. You should get video on startup. You can go into BIOS and disable your onboard video then.
Go into windows and make sure you have the display drivers installed and everything looks ok.
You should be able to right click on the desktop and go to properties. Go to settings and then advanced. This has all the options for your mx440 now. Depending on the driver version you have there should be a displays tab, or an mx440 tab or something... I haven't used an nvidia card in over a year so I'm not sure what has changed. Anyway, somewhere in this area you should be able to select a "switch amongst displays" or "clone" mode. This will allow you to send output to the projector while in Windows. Switch amongst displays will enable the projector and disable the monitor. Clone will show the same on both screens. (Clone mode wont be able to show motion video on the projector, but if this is just for text or pictures or something, it should be fine). There will also be an extend desktop or something, which is not something you'd want probably.
I hope this gets you started.