I only just found this topic and wanted to add my own thoughts.
As to the question of whether Germany could have won WW2 I think that depends on what you mean by "won". Could Germany have conquered Britain, the Soviet Union, and the Middle East? I have great doubts about that. However, the notion of Germany going on to dominate Europe for an extended amount of time strikes me as far more plausable.
As I see it Germany did have a good chance to be ultimately "successful", but its odds weren't as good as it seemed at the time and required them to do most everything right. There were a lot of decisions made earlier in the war that pretty much doomed Germany later, but if other choices were made it could potentially done much better.
For example, if the force at Dunkirk had been destroyed or captured rather than escape that would have given the Germans greater diplomatic influence over Britain. Faced with no army and stinging from a terrible defeat the Brits might have gone with someone else rather than Churchill and sued for peace. This might have happened anyway if the Germans had stuck with destroying the RAF during the Battle of Britain. It wouldn't necessarily have meant German occupation, but it would have consigned Britain to being a virtual Nazi satellite. More to the point, it would remove any real threat of a two-front war once Hitler went after the Soviet Union.
Without Britain stirring up things there might easily have been no reason to delay Barbarossa with an invasion of the Balkans. Those extra five weeks could have allowed the Germans to take Moscow. I don't think that would have caused the Soviets to surrender (if they could keep going after the disasters of that Fall they could after losing the capital), but it certainly would have prolonged the war. Moscow was not just a symbol, but also a vital manufacturing center and railroad hub.
I agree that aid from the US and Britain wasn't the deciding factor in allowing the Soviet Union to survive, but you can make a much better argument that it did give the Soviets the extra resources they required to ultimately win. Take that away and you might have seen a prolonged stalemate in the East leading to an eventual ceasefire of some sort that would have preserved both governments. Keep in mind that by the end of the war even the Soviets were scrapping the bottom of their manpower barrel. Add in a few extra years of combat and I don't believe either side would have had the resources to defeat the other.
Things could have also been a lot different if Germany hadn't made a couple of basic mistakes. One already discussed was Hitler declaring war on the United States for no particular reason -- absent that Roosevelt would have had a hard time convincing the nation to independently declaring war on Germany rather than just concentrating on destroying Japan.
Another is Germany's failure to really capitalize on its great economic potential. By the end of 1941 that potential rivaled that of the US, yet Germany still produced less than either Britain over the Soviet Union, two economies only a fraction of the size. It wasn't until 1943 that the Germans began to really approach their economic potential, but by then it was too late. If they had started the process two or three years earlier the war would have likely been years longer -- perhaps enough for the Allies to prefer a negotiated peace rather than unconditional surrender.
So, with some important changes to history you might have ended up with a Nazi dominated Europe, although Britain and the Soviets likely would have survived. In that case I suspect that the Cold War would have been with them and that in the end the German empire would have collasped in the same manner as the Soviet one eventually did. However, I think it is clear more people would have died in the process. As terrible as Stalin was he was still preferable to Hitler, IMHO.