I am having a bit of trouble finding a definitive answer for this. It seems Unreal games above a certain engine version do not officially support Windows 7 anymore, but sometimes times will run and sometimes won't. The problem could also be linked to other PC troubles. If that is the case there seems to be a bunch of possible solutions when you google the problem. Looking over some I would say make sure to update your GPU drives if they are not yet. If I find any solution I will let you know.