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
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.