Select Data in a table and calculate a field from another field using modelbuilder

I have been trying to build a model that selects data in a Shapefile and calculates one field from another field. The Calculate tool works fine but how do I first select the data I want to calculate? The Select Data tool doesn't work that way and The Extract\Table Select tool puts the result in a separate selection table and the any calculate takes place in there and doesn't affect the original table.

I need to be able to do this weekly on a number of layers.

Do I need to write a python script?

I'm using ArcMap 9.3 with an ArcInfo license.

Thanks,

DJC
City of Tallahassee, Fl