Formula for working days

All,

I am wanting a formula that will populate a column automatically with working days, as a number when I enter specific dates.

EG

I enter into

Column A: 6th September 2010

And into

Colum B: 01st October 2010

Column C automatically populates with the Number 20 - 5 working days per week (Monday to Friday)

please help

thanks,

Moggzy