Audacity with RealTek Audiomixer
Using Audacity with RealTek Audiomixer on Windows Vista may need some tweaking.
Check in the system tray, the Audiomixer settings:
right-click on the speaker icon in the system tray and select volume control
right-click on the speaker icon in the system tray and select sound manager (you may also have access to these settings through the control panel)
check the recording settings and select the Stereo Mix as input device