Video card for live video switching?

I am setting up a system for our school's auditorium that will be used
to project various forms of media onto our rear-projection screen, and
I'm looking for an ideal combination of software & hardware that would
accomplish it. (we have the projector permanently mounted, and only 1
VGA input on it)

We need to be able to playback DVDs and VHS tapes (from an external
source, so I assume via video capture / passthrough to VGA???), as
well as powerpoint, avi, mpg, etc.. files. I would like the interface
to appear, on the screen, to be relatively seamless switching between
inputs / applications (ie. not having to mess around with windows
start menus, explorer etc. too much.)

What video card / software / or even external hardware devices do you
think would be best...?

Thanks for your advice!

David Hoare