How would you describe the sound when its at 0%? Is it just noise like a hum, or is it like its not at mute and you can still hear audio?
On your sound card, the green socket is for passive speakers. Generally this can get up to about 50W, and is advised not to be put through to an amplifier. You should use the Line Out socket instead, (blue, if memory serves correct). This will give a line level output which is nicer to amp.
You should then turn the Windows volume to around 90-95%, and play some normalised audio like an MP3 or CD. Next, turn the speakers up to as loud as you want the maximum to be.
The speakers are now your master volume output, so you shouldnt need to touch this anymore.
Now turn your Windows volume down to 0%. Do you still get any noise or audio?
If your using the Line Out port on the soundcard, you shouldnt hear anything.
The reason we went to 90-95% is for those days you have a good song, and want that extra 'boost'. Also, you shouldnt really have the Windows volume at 100% for extended periods of time, as this runs too much gain into your amp.