I'm sure this has probably been asked a number of times, but....

My game keeps freezing even though I am playing it on a 3 GB computer. I can't understand why this is the case unless it's actually the game. Anyway, I have heard that playing the game in windowed mode helps to increase the game's performance so I went into preferences and went to change the display to windowed but it is greyed out and unavailable to select. I've turned off the Stadium and Sky settings which I've also heard is useful but why can't I switch to windowed mode?

Thanks in advance.