How do i stop post back. for example i have two textbox's that i am adding together and put the result in box three. but i don't want it to post back?

You mention 3 textboxes, then you say "i (sic) don't want it to post back".
To what does "it" refer?

If you could give a little more detail on the actions being taken, what the undesireable behavior is, and what you would prefer to have happen, that would really be helpful.