We now know that the UK was passing false information to the Soviets indicating that the French and Germans were in the process of negociating a separate peace that would allow the Germans to turn east vs. the Soviets. Without this, I'd say Stalin probably stays out until Germany is exhausted - and considering how badly bogged down the German advance had become by October, I'd say it's unlikely the Germans could have knocked France out of the war. So, Stalin goes west in '41 or '42 rather than '40, and possibly we end up with the Red Army on the Rhine.
(BTW, it wasn't the Red Army that overthrew Stalin. It was a party coup headed by Beria - it was only later that Beria was in turn overthrown by a party/army alliance vs. Beria's secret police, when he was unable to impose his authority on a national level. The army was too firmly under control of the Party to mount a coup on it's own in Stalin's day, although after the events of '44 it became rather more autonomous).
The irony of it all was that Stalin went to war to prevent an entirely imaginary anti-Soviet entente which he ended up creating in fact - after Hitler's overthrow, the French and the British were willing to work with the Germany military to prevent the Soviets from taking central Europe, although Germany had to withdraw from it's western conquests. Getting roughly Russia's 1914 western borders in the '42 armistice hardly compensated for creating a solidly anti-Soviet European alliance.
Bruce