Finland won the Winter War by losing it as little as was realistically possible. I personally believe the OTL result was firmly among the best 20% of all the possible ways the war could have ended for the Finnish people.
Now, I guess with a right POD one could concoct a theoretical war that sees a Soviet Union attack a Finland in 1939, one that leads to this Finland not losing land to the USSR in question. But making it a war where Finland and a recognizable USSR are the only participants would be a decent feat.