I use MS Excel 2007.
I have data validation list for a range of cells. There are 4 values to
choose from.

My requirement: There are 2 values which, when, selected should prompt the
use to enter a text/comment that is saved. This comment/text can bee seen by
moving the mouse over the cell.
However, if the value is changed to either of the other 2 - the comment/text
is automatically deleted.