They make adapters that will allow your threaded system to accept a threadless stem (just google something like 'quill threadless adapter'). If you want to go to a full threadless system you would need a new threadless 1" fork, a new threadless headset, and a spacer for your stem (most new stems are for 1 1/8" forks).
Edit: the other, most obvious, alternative is to just get a new quill stem to replace that adjustable one.
Last edited by Quel; 01-19-10 at 10:34 PM.