Good advice guys! BUT... if you are determined to create a computed column in a table, I believe SQL7 and SQL2000 will allow you to do so without a trigger. Here's a sample script that works in SQL2000:
create table table1
(myid int,
value1 int,
value2 int,
comp_col as value1 + value2)
The thing to understand about computed columns is that the value is generated on the fly when accessed. This is different from an insert trigger which calculates and stores the value when data is inserted and/or updated.