I have a Task Account

I use lookup feature with a category list on a separate sheet. OK, so
far.
Right of the category drop down list I have drop down list to choose
amount hours. The hours are then totalized. How to have not the value
totalized when a certain category is choosen?

Sample:

CATORY: HRS:
Finance            3
Sales                5
Off                    2
Training            1

TOTAL             9 as OFF is not included.  

Bart
Excel2003