I strongly feel that the inability of the Germans to capture Moscow was the turning point. And in order to have a chance, Moscow would have needed to fall before December. Micheal brings up an excellent point that the German's had outrun their legs by that point, but had the German's not elected to envelop Kiev and instead used that time to rest and refit, they certainly could have taken Moscow with a decent chance of being able to hold it through the winter. The Soviet Union would have been effectively cut in half.
I doubt the Soviet government would have fallen with the capture of Moscow, but the blow to morale would have been huge. Soviet forces were already having issues with low morale, and losing the capital might have been a mortal blow to the Russian army's (or large chunks of it) will to fight... or might not have been. The German's would still have been overextended, winter would still have arrived, and (without the Kiev envelopment) they would have enormous Soviet forces in the south to contend with. Personally, I don't think the German's could have pulled it off anyway, but I do think it was their only shot. To paraphrase Glantz, the Germans would no longer be able to defeat Russia after their failure to take and hold Moscow.
Reed