Whenever you open multiple instances of the same application in Windows 7 it will stack the taskbar icons one behind the other so they are (pretty much) hidden. In order to switch between them, you need to hover the mouse cursor over the icon to reveal the various thumbnail previews and then select the one you wish to change to work with.
[attachment=2:2mtrqbzw]Always combine, hide labels.png[/attachment:2mtrqbzw]
[attachment=0:2mtrqbzw]thumbnail previews.png[/attachment:2mtrqbzw]
I can understand why Microsoft thought that might be a good idea as it helps prevent overcrowding the taskbar and filling it up too quickly/easily. But it is also an inconvenience, an annoyance