The Russians had many different concerns at the outbreak of the Cold War, but to say they were afraid of the Americans might be slightly inaccurate.
When World War II ended, the country they were most afraid of was Germany. They had been invaded twice by the Germans in less than 30 years, and both times the effects on their country were devastating. Millions of civilians had died along with the soldiers and the countryside was ruined from scorched-earth tactics and prolonged fighting. In order to ensure that they were never attacked again, the Russians kept the nation divided at the same time they began exerting political influence on neighboring countries in an effort to guarantee them a secure boarder. I think its safe to say that this was the Russian’s main fear.
A secondary concern would definitely be the power and influence of the U.S. They were supporting many of the countries in western Europe with money and supplies through the Marshall Plan which threatened to undermine Russian efforts to control their satellite nations. Also, the nuclear prowess of the U.S. was a worry since a Russian nuclear devise was several years away.