Trying to make a custom "blank" key for Ventrilo Activation

So I am trying to find a key that has a registry within windows but is not used for anything else so that I can get my razer copperhead button to work as a speech input bind in ventrilo. If I use "Windows key 4" or "Windows key 5" they are bound somewhere within windows to act as "back" and "forward" in web browsers. Optimally I would like to unbind one of those keys, since they seem to have no alternative purpose, but how would I go about doing so?

Hmmm I have a Razor Copperhead and in Ventrillo I can assign my mouse key to toggle the mic without doing anything.