The traditional approach would be to create an INSERT trigger. Starting with v7.0, SQL Server supports computed columns.

CREATE TABLE foo (
c1 integer,
c2 integer,
c3 AS c1 + c2
)

The thing to understand about computed columns is that their not materialized. The value is generated on the fly when accessed.

The approach to take depends on what you're going to do with the column. And I guess the other question is "do you really need to store the calculated value?"

-Mike