I am actually not entirely sure you can switch to a window not spawned by the driver. I think the people working on the selenium 2 project have worked a little bit on switching to a window spawned by a different driver. When you do:
driver.getWindowHandles()
All you get are the windows spawned by the driver object it is called on.
This is a pretty old feature request: Allow webdriver to attach to a running browser . So it's not possible right now.
