Struggling to record Audio played on the computer

Hi

I'm struglling to rcord the audio that my computer is playing (ie ripping audio from youtube)
In 'Preferences' under ' Playback' i have two options-
Microsoft Sound Mapper-Output or
Realtek HD Audio Output


and under 'Recording' I have-
Microsoft Sound Mapper-input,
Realtek HD Audio Output or
Realtek HD Digital Output

and none of teh combinations of these are allowing me to record the sound that i am playing on my computer, is ther some thing big i'm missing? I set it up to work last year and it was fine, but since then un-installed and re-installed Audacity and can not remeber how i set it up last time :/

Any help would be greatly appreciated