asp.net controls open new window!

I have a javascript button which is opening an aspx page in a new
window (using the showModalDialog Method). The window size is specified
by the javascript, also specified no menubar etc. So far so good.

When I put controls on this aspx page (i.e. the new window), whenever I
click on them they render in a separate new page (fully and normally
sized page). Of course I don't want a new page to be opened, I want
everything to rendered in the same window.

Any ideas of what I can do to make sure the controls 'do their thing'
in the same page.

.

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