So, here are a few scenarios:
1) On 1 October 1939, Stalin orders the Red Army to move from operations against Poland to operations against Germany as well, since they're on a roll and Hitler is a lying weasel who you wouldn't trust as far as he could throw you.
2) On 14 March 1940, after the surrender of Finland, Stalin's ears twitch and he suspects that the successful conclusion of affairs in Finland may be only the beginning, and orders planning for a campaign to the West. On 15 May 1940, with the surrender of the Netherlands coming on top of other French defeats, Stalin becomes paranoid that Hitler is just going to win in the West and then turn East, and orders the Red Army to implement the plan and attack west. D-Day is scheduled for 1 June.
3) On 30 May 1941, Richard Sorge advises Moscow that Germany is going to attack the Soviet Union by the end of June. Stalin happens to be looking at a picture of von Ribbentrop when he is briefed by Beria and is so irritated by the German Foreign Minister's image that he decides to believe Sorge's intelligence, and orders plans for a counterstrike, to be launched on June 20 to give time for the rasputitsa, the time of mud, to dry out.
I think 3) will be a disaster for the Soviets, leading to earlier encirclements than OTL and greater losses overall. Moscow may even fall, which will at least severely curtail Soviet warmaking ability, if not cause it to sue for peace altogether.
2) is probably enough to keep Mussolini out of the war and the French in it. Initial Soviet performance will be poor, with the officer corps, inventory, and logistics train all going through reshuffles at the time the order is given. Casualties will be high, but the Polish occupation forces will retreat towards Germany (no fight-to-the-last-bullet Fuhrerbefehlen at this stage). This proper two-front war will see Germany sue for peace (possibly after Hitler shoots himself shaving), which the Soviets may accept (after they annex all of Poland), but the French and British will not, as they are thinking about phrases like "so that no stone stands upon another" and "plough the fields with salt".
1) is unlikely because it's such a bad idea, as France and Britain aren't in a position to really press the war in the West yet, and Belgium, the Netherlands, Denmark, and Norway aren't involved. Germany can hold off further French advances into the Saar while keeping their eyes on the prize in the East, and will probably rout the RKKA in the field after an initial getting-to-know-you period. Everything bogs down in the snows a bit east of the prewar Polish borders, and in the spring the Entente is ready to play. The war ends in late 1942 with French troops in Berlin and Soviet ones in East Prussia..