How to prevent postback on button click.

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I have this one window which is a webform for a service request. On the form are buttons that will open another window to allow the user to make choices which will then get returned to the opening window.

The problem is that unless the values are returned to textboxes the returned values will get wiped out when you click a button to open another window. Such as the HTML that I return to a label for display purposes.

Now the problem is that when the button is clicked it posts back to the server.

Can anyone tell me how to prevent postback on the button click.

Thanks.