PP, there's a whole forum dedicated to Winter Cycling>
http://tinyurl.com/y8p62pb
If you're going to put on a set of studded tires, leave em on until the snow & ice have moved on. There's no point in changing them when the weather folks say it might snow or if the temps hover around freezing.
In addition, here's a site that has plenty of info about studded tires>
http://www.peterwhitecycles.com/studdedtires.asp
Enjoy riding during the cooler months!