Make a count based on Drop Down Option
 

I am currently working on an error log using an excel 2003 spreadsheet. Where
the person using the sheet selects a drop down menu they can chose a variety
of problems. What I would like to do is for what ever they pick it will add a
1 to a running total. I also want to make sure that the counter will be
accurate incase someone marks something it will add the 1 but if they change
their mind or chose something else it will take the 1 away from the total
that the 1 was added to before.