Combining multiple Videos
On Windows, for free, your best (albeit complicated) bet might be Blender. 

You could create a series of several plane meshes in the layout you wanted, in front of the camera. You'd then create a separate material for each of the planes, setting them to shadeless, and giving each a different video texture. You'd have to mix the audio separately (Windows Movie Maker, or Blender also has a built in video editor). 

I'll note that using a 3D approach for something like this isn't ideal at all - a 2D compositing program like Adobe After Effects would be much better (Blender itself does have a 2D compositing component, but since it uses nodes instead of layers, it might actually be more difficult).