Some other application, or Windows, is automatically changing the level.
Can you try going to Control Panel -> Sound -> Recording, selecting the microphone and clicking 'Properties' then select 'Advanced' and try changing the 'Exclusive mode' settings.
You can also go to the Communications tab and set 'Do nothing'
If none of these work, try googling the problem - other Windows users (not FBX users) have had the same problem.