In the Tools – Options – Environment – General page, you’ll find the checkboxes

  • Close button affects active tool window only
  • Auto Hide button affects active tool window only

Below is a snapshot of two tool windows docked together with the "closed button" and the "auto hide button" circled.

Solution Explorer and Class View tool windows docked together

If you want just the solution explorer to close when pressing the ‘x’ button, enabled the option.  Otherwise, both tool windows will close.

If you want just the solution explorer to auto-hide when pressing the push-pin, enable the option.  Otherwise, both tool windows will close.

Technorati tags: VS2005Tip, VS2008Tip