In Windows 7, Vista or XP you can try to enable your Stereo Mix (stereo mixer) from the Control Panel, by updating drivers and installing alternative drivers:
Missing sound recording option "Stereo Mix" / "Record What you hear" / "Waveout mix"

Please note that for some computers there are no drivers enabling the Stereo Mix. And without such a drive, you can't just enable it in the control panel of Windows 7.

But there other recording options in Windows 7. You can use software that doesn't need the Stereo Mixer for recording (many programs are compatible with Windows 7):
Audio/video recording software that allows to record sound without WaveOut/Stereo Mix (Windows 7, Vista, XP)
Please note that there are sound recording programs, video capture programs that can record without stereo mix, so you will probably be able to solve all of your Windows 7 problems.