Need to record audio internally, not through the microphone

There are a few specialty tools available that help to record stuff from your PC. However, you should be able to do this using a built-in Windows setting. It's a little hidden:

1) Right click the speaker icon in the taskbar and select Recording Devices.

2) Right click the empty space in the list of devices and select Show Disabled Devices.

3) A new item (if it wasn't already visible) should appear called Stereo Mix. Right click this and select Enable.

Now make sure you have an audio recording tool installed. Audacity should do the trick.

1) In your audio recording tool's settings (Preferences in Audacity) select your recording device input. Pick Stereo Mix.

2) Click the record button in Audacity, then play the file on the PC that you want to record from.

3) Stop the recording when you're done and edit to taste.

4) Export as the audio format of your choice.