based on: http://mattberseth.com/blog/2007/10/modalpopup_with_multiple_cance.html
1. Add BehaviorID to the Popupextender:
<cc1:ModalPopupExtender BehaviorID="mdlPop" ..
(from Ajax.asp.net: BehaviorID: In cases where you would like to access the client-side behavior for your extender from script code in the client, you can set this BehaviorID to simplify the process.)
2. Add code to the cancel buttons:
.. OnClientClick="$find('mdlPop').hide();return false;" ..