Transaction Advice Anyone?

[jfer]

I am implementing a form for a web application that is being converted
from a legacy fox pro application.

Basically the form objective is to loop through every row of a
particular table and set a flag on the row to a particular value.

The fox pro application locked the entire table prior to starting the
work. If I create an SqlTransaction with a command attached that
performs a SELECT to read in all the data will I effectively lock the
table the rows came from while the operation is working?

Also what happens if while the first command has the data read I attach
another Command object that SAVES each row as they complete. Won't the
first READ operation block the UPDATE from completing?

I've been reading about transactions all day, and have used them but
never in a scenario like the following.

Thanks for any insight.