Need server control help...

[Suresh]

Peter,
Any browser based pop-up windows can only be opened with
JavaScript.

See
http://developer.irt.org/script/window.htm
on how to open new broswer windows using JavaScript.

You can still accomplish this using the server side button
in your custom control.
On page_load of your control add the following,
myButton.Attributes.Add("OnClick", "JavaScript:window.open
(yourparams)");

HTH,
Suresh.