Categorically disagree with VNC session sharing. I just tested that myself to verify. The Windows TightVNC package's VNC server provides for "automatic shared sessions" in its "Advanced" settings.

And given the description by the OP, he's definitely looking for shared session. Otherwise, I'd recommend Terminal Server or something. Or some Linux with remote X. But somehow I doubt that's within the realm of possibilities.

Edit: really, there's no need for bashing Microsoft's OS, pathetic as it may be. If three people need to work on a single PowerPoint presentation, or worse, AutoCAD diagrams, you don't have much of a choice but to use Windows.