configuring recording devices (Stereo-mix)

Hello.
I'm having some difficulties configuring audio on my PC running Vista
home premium.
I've figured out how to unhide the Stereo-mix recording device, so I'm
able to record my voice from a microphone as well as system sounds.
However, I'm not able to control the volume of audio on the computer
to ensure the audio recorded via the microphone is at the same level
as the other sounds in the computer.
Can anyone explain where exactly in the audio settings I should
control the levels of various audiosources on the computer, so I'm
able to turn down the volume of computer sounds while maintaining the
same level of the microphone.

The problem is that when I record things, the audio from the computer
is often too loud in relation to the audio recorded from the
microphone.

Kind regards and thanks in advance for any suggestions, Niek