How do I make MSAccess97 add or subtract?

I am making a virtual check book. So when the user enters in a check amount, they have to click either "deposit" or "deduction". When deposit or deduction is pressed and the database is updated I need Access97 or VB to look at the new entry and the "deposit" or "deduction" and add/sudtract from the existing balance. How do I make MSAccess97 add or subtract an entire column of currency entries?

Any help is greatly appreciated,
Daniel Christie

Update MyTable set MyColumn = MyColumn - MyValue [Where condition = true]