Stereo mix?

[Finska]

I've been told about stereo mix which helps you to record your microphone sounds and computer sounds. For some reason, I don't have it, so I need to record my videos with headset. I select recording to "microphone" in Windows' settings and in Fraps' settings I select "stereo out" so I can record game sounds and micsounds with "stereo out".
It gives me a problem that I can't talk in skype while I'm playing, because my gamesounds are heard in chatpartners computer, LOUD. That's a big problem and I wanna fix it. What can I do? Do I buy new audiocard or?