How do I open two "Computer" windows on Windows 7?

Click with the middle mouse button in the taskbar on a Windows Explorer button.

The simplest way is to press Win+E (thanks @Abraxas), or Ctrl+N whilst in the Computer explorer window (Or any explorer window you wish to duplicate). You can also click File -> Open new window (This is in Windows 10, the option may be slightly different in Windows 7)

Alternatively, if you wish to do this by a shortcut, set up a shortcut that opens explorer.exe /n, or if you specifically want to open Computer, set the shortcut to %SystemRoot%\explorer.exe /n /e,::{20D04FE0-3AEA-1069-A2D8-08002B30309D}, depending on the behaviour you want. This will always create a new window.

Quickest two ways:

Win+E that will open up a new explorer window.

or

Middle-click on the Explorer in the Taskbar

or

Ctrl+N when in an open explorer window