Are you looking to learn how to windsurf, or are you a seasoned windsurfer looking to get out on the water?
If you are a seasoned windsurfer then you have come to the wrong place!!!
There can occasionally be some okish conditions here in Switzerland but rarely anything epic. The nearest place to rent to Zurich would probably Urnersee www.windsurfing-urnersee.ch but the guy there is a complete a$$hole, we went there to rent a few years go and he made it seem like he was doing us a favor to rent us gear! Conditions on the lake can get some thermic at this time of year if your lucky maybe a force 4. If there is bad weather on the south side of the alps and good weather on the north side then you get a wind called Föhn in the lakes around Urnersee, this wind can get quite strong up to force 8, but is unbelievably gusty.
Silvaplana is another potential windsurf location http://www.surfclub-silvaplana.ch/ it is a really cool place to windsurf at 1800m up in the alps, right next to St. Moritz. The scenery is amazing and the wind can be ok, there is a school and a rental center here also. Due to the altitude the air is a little thinner so the power of the wind is reduced by about 1 force. Personally I prefer it here to Urnersee.
There is also a rental center in Betlis near Weesen on Walensee, I don't know much about it but the area looks really nice.
Other lakes of interest are,
Neunebergersee for Bise and Southwest wind, can be quite good fun. Murtensee, www.bisenoire.ch again there is a school and rental centre here.
Bodensee (Berlingen), good in southwest wind, (only 1 hour from zurich) there is a shop and rental place there called boarder house if you google them you will find contact details.
Lake Como, Italy, Can get thermic this time of year, and a wind called NordFöhn when the weather is bad on the north side of the alps and good on the south side. There are rental centers in Domaso and Collico.
If you need anymore info about windsurfing in Switzerland just get in touch. Good conditions are very infrequent and usually there is no rental place nearby so it is usually better to have your own gear.
My tip is weekends in Silvaplana, but be prepared to do some biking or something else also.