Why can't I convert a QT video clip to .wmv?

I got the free Qucktime 7.7 and then purchased QT Pro so I could convert QT vid clips to wmv. However, I am not given the wmv conversion choice on the QT Pro drop-down screen. As a result any QT vid clip I save as a new new clip is missing the video during playback; only the sound can be heard. How do I correct this?