If you drive in snow and need the traction then it can help to turn the traction control off, but otherwise leave it on, especially when its wet. I would also read the threads on winter tyres, useful it is
http://www.z4-forum.com/forum/viewtopic.php?f=49&t=30740&hilit=winter+tyre+thread
http://www.z4-forum.com/forum/viewtopic.php?f=49&t=30740&hilit=winter+tyre+thread