Actually you're wrong. You always had an option to use a different OS. You could even purchase a computer that didn't run Windows.
The courts decided that, since Microsoft had an overwhelming majority share of the desktop market, they should be considered a monopoly. They abused their monopoly by not only preinstalling internet explorer and making it an part of the OS but also by restricting the OEMs ability to preinstall a competing browser as a condition to receive heavily discounted licenses per machine sold.
Despite the actual motives behind Russia decision, the argument used against Google is very similar to the ones made against Microsoft. Google has a large share of the wireless handset market and it is alleged that they require manufacturers to make Google the default search engine. If this is actually the case then I can see why people liken this to the US vs Microsoft case.