Create a callout box/Context menu for a cell

I want to create an input box that is triggered off a user entering data into
a cell. I want the box displayed after the user hits the return key. I am
assuming it would be triggered off the calculation event.

I would like to have the input box look more like a callout that extend off
to the side of the cell (instead of a msgbox). It might be good to build it
around a context menu as well.

Is there an elegant way to do this in VBA? Or do I have to use VSTO?

Thanks

EM