On Windows, you can choose the Windows WASAPI Audio Host and then the (loopback) input in the Recording Device box. Choose the loopback input for the computer playback device you will be listening to (for example, "Speakers (loopback)". The loopback input records computer playback even if your sound device lacks its own stereo mix or similar input.
Windows, computers almost always only have microphone inputs enabled by default. Earlier Windows systems may also need the input for recording computer playback to be made visible before Audacity can use it. To show or enable inputs, launch the sound device control panel from the Windows Control Panel or from the system tray (by the clock).
If you still have no stereo mix input in Windows, sometimes this input can be enabled in the audio interface's own control panel, especially with older RealTek devices. The audio interface's own control panel can be found in the Windows Control Panel.
In the settings menu of the window manager xfwm4 under "Accessibility" you can check "Hide title of windows when maximized". This is of course very useful in combination with the xfce4-windowck-plugin.
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