Microphone volume issue with D100 and Savi 400 on Windows XP

I use a D100 with a Savi 440 headset on a Windows XP system. 

I can change the microphone volume of the D100 using the Windows volume controls, but changing the volume does not have any effect on the recorded audio (0% and 100% produces the same audio).

Should I be able to control the microphone volume like this and if not, is there a way to control the microphone volume?

Thanks,

Heinz