That would depend on what you would consider "defeated". Knocking out active military capability is somewhat of a shot in the dark. Based on some reflections after looking at the critical aspects of the offensives in 1942, John Mosier came to the conclusion that they should have pushed for Moscow (Army Group Central should have been priority) because it was the hub of the nation, but it's difficult to say whether or not it would have knocked out the USSR as a functioning military entity, for a variety of reasons. First is that not all of the industrial capability was centralized around Moscow, second was that the USSR was receiving a fairly significant chunk of planes, tanks, and trucks from the United States, and third is that guerilla war was definitely a possibility.
I fear that I might veer into the ideas of hypothetical here, so I won't stick too long on it, but from anything I've read, I cannot say that the USSR was close to being defeated in any (lasting) capacity. Accounts from German soldiers that were published later on showed that while they were winning in 1941 and 1942, there were several cracks in the armor, so to speak.
So while I don't believe anyone can say for certain, and without going too far into what we don't know, I would say that the USSR wasn't close to being defeated. They had lost several defenses, but with the industrial backing of the United States, as well as their own functioning industrial capacity, combined with the attitude of the Red Army (the whole not one step back thing), and the fairly crudely prepared German forces for winter all seems to point to an eventual Soviet victory no matter how you look at it. The only question that can't be answered is what if Army Group Central over Army Group South?
I would recommend reading Reader's Digest Illustrated History of World War 2 for a lot about this. Great primary sources from soldiers and officers on the Eastern Front. John Mosier wrote The Cross of Iron, Rise and Fall of the German War Machine and I personally found that quite informative (in the sense that it made me think very differently about the war), but I'm unaware of how his book holds up to scrutiny.
That is a very difficult question to answer, for a few reasons.
Firstly, which factors do we take into consideration? Obviously the USSR wasn't defeated, so to some degree we are already speaking of hypotheticals if we want to determine which factors that, when tweaked, would yield a different result. Do we only take into consideration that which directly concerns the Soviet Union itself? Do we look at what the Germans could have done differently? I will be focusing on internal Soviets factors, although some German factors are unavoidable. What I won't be doing is entertaining "what if" ideas (What if the Germans had focused on Moscow? What if the Germans hadn't been encircled at Stalingrad? What if the Germans were better prepared for winter warfare? You get the idea). This answer isn't about what the Germans could have done to defeat the USSR, but how close the USSR came to defeat in reality, and why.
Secondly, as /u/HaroldSax says, what does "defeated" mean? A complete military defeat? The collapse of the political system? The former seems unlikely in the extreme. The Soviet Union wasn't just a geographically large country with large untapped resources, it also had a much larger population than Germany. And those two combined in part with the Stalinist system meant that the Soviets could and did move large parts of their industrial base beyond the reach of the Germans and continue to supply (with ample Allied assistance) a growing military. Evan Mawdsley argues that the German economy was not really geared towards "total war" before the winter of 41/42, which was likely already too late. Contrast this with the sort of "anything to be victorious" structural flexibility that was already built into the Soviet system.
That last point plays into the latter of the two main points, which was the potential collapse of the political system. Here it is important to involve Germany more directly. An inherent quality of the Nazi system was its belief that the people(s) of the Soviet Union were largely inferior to their own, and they treated people in the conquered territories as such. This meant that instead of playing on the very real tensions that existed within the Soviet state, politically and ethnically, the Germans actually strengthened the Soviet sense of comradeship and unity. The name "The Great Fatherland War" isn't all propaganda. There really was a genuine sense of comradeship through massive adversity (just look at Leningrad). Add to that the fact that the Soviet leadership bar Stalin had already evacuated Moscow, and it makes it difficult to argue that the capture of Moscow would cause the entire state to collapse. It would certainly complicate matters, but then matters were already fairly complicated.
To sum up: the Soviet military was unlikely to ever be completely defeated on its own, with the backing of a growing industrial base and allied supplies. That backing wasn't going to disappear unless the state itself was compromised, and I can see little reason to believe that the political system was close to collapse, even with the Germans parked outside of Moscow and elsewhere. Some historians actually argue that Germany lost the war as soon as they started it, but all I will commit to saying is that I do not believe the USSR was ever really on the brink of defeat. It was large, populous and increasingly united.
Sources (these are the ones I could find lying around right now):
Evan Mawdsley - The Stalin Years
Edward Acton - Russia: The Tsarist and Soviet legacy