Calculate number of business days excluding weekends and holidays?

I have an excel sheet with 3 columns-

start_date end_date No_of_Workdays
7-jan-08 14-feb-08
13-Jan-08 23-feb-08
15-feb-08 13-Mar-08

I have to calculate the number of business working days between start_date and end_date in Excal VBA programming.Can anyone tell me the what logic should be used here? If someone had done it earlier , please share the code.
One thing more,can i use Networkdays() function in a loop.Because i want an automated solution.If some day i change any date ,then the difference should be reflected instantly.