entering values into a cell
 

I would like to know if I can prevent a certain value being entered into a
cell if the preceeding cell has acertain value. For example if cell 1A has
the value N entered in it can I prevent the value E being placed into cell
1B, obviously if any other value is in cell 1A I want to be able to enter the
value E. This is not a validating issue as I am also using validation rules
so that only certain values can be put into cells.
Thanks in advance