Is it a way to stop IE to create a new instance everytime it encounters a popup window? I'm debugging two Silverlight applications at the same time. The first one creates new popup window with the second one. Debugging works fine in FireFox because it does not create a separate process. But IE creates new instance and of course Visual Studio is not attached to the new instance of IE.
Which version of IE? If you are using 7+ can you change the popup setting to "Open in a new tab" instead of a new window? (Tools -> Options -> Tab Settings).
As far as I know, when you open a new window in IE (either by using Javascript, or selecting "File > New Window"), it should not start a new process. A new process is only started when you actually run the executable again (from Windows explorer, for instance).
Since MSIE is so closely related to Windows Explorer though, I wonder if it's somehow tied to one of the Windows Explorer settings. Try going to any explorer window, select Tools > Options, select the "View" tab, and make sure that "Launch folder windows in a separate process" isn't checked, that may be it.
Another possibility is in MSIE, Tools > Options, select the "Advanced" tab check "Reuse Windows for launching shortcuts," though I doubt this would be it.