Set up your sound card and Windows Mixer

The exact procedure and menu items for doing this will be slightly different for different computers and versions of Windows, but the general idea is to 1) enable recording, 2) select the correct input, and 3) set the volume control. Here are some more details:

Go to Start | Programs | Accessories | Entertainment (or Multimedia) | Volume Control - or - Right-click the speaker icon at the lower right of your screen - and open the volume control.

Click Options | Properties, and click Recording or "Adjust volume for Recording".

Line* or Mic/line* should be selected and the volume control should be up. If you leave the recording control panel open while you record, you should see the level indicator to the right of the volume slider move up and down with the sound.

*NOTE: On some computers (mine, for instance) you will need to select "Mixer" rather than "Line". You might have to experiment. Connect your music source to your computer and start it playing while you try various inputs. When you have found the right input, you will see the level indicator (if your computer happens to have one) move with the sound. You should also hear the music from the computer's speakers.  Then you should un-check the other recording devices to prevent noise from those inputs.

Then you can close that window, although you might want to come back here later to adjust the input level.