The ASVZ is the sport club of the university of Zurich. I quite sure they also offer windsurfing classes - you can check out their website (in german) if you like.

I spend a lot of time diving, and they have an office right next to the dive site - they offer all sorts of water sports.

[Disclaimer: I have verified that the above link is correct, though forum users should always exercise caution when clicking on external links, lest they lead to unsavory websites and result in getting you fired]