Try Alt+Enter when you open it, this way it'l probably go fullscreen
Other option, Right click on US 2 from your steam library, click on properties then open " Set Lunch options" Then type in "-fullscreen" without the ", then click on Ok and try lunching it
>> You can also do the same thing, but this time, Properties > Local files > Verify Integrity Of Game Cache
This will clean up any bad-files and re-download them again