This describes exactly what happened to me as well, went back to main menu clicked x, came back a few hours later and kept getting that error when it tried to check content after title screen, hasnt worked for 24 hours now. I did try what you suggested but fullscreen was already turned on...Just so everyone knows I believe everyone iv seen that cant fix this error has had an intel CPU and nvidia GPU. look on steam forums and you'll see tons of people that are stuck like I am. Im running i5 2550k gigabyte GTX 670 win7 tried updating video driver didn't help. Its just very strange all these people are finding a fix by opening up C: drive space. I had 40gb free, made it 60gb free, tried moving to 2nd drive with 150gb free, tried adjusting pagefile, still nothing. Even more strange is I'm the only one iv seen that played before it became unfixable I'm pretty convinced (and honestly jealous) that i am no longer in control of when I can play next.