I'm running this game on my Windows 7 Ultimate 64-bit, Phenom II X6 1100T, HD 6850, 8GB RAM and 1TB HD PC with DirectX 11 via Steam. I'm using Bitdefender and I'm sure it's the main cause of the problem, changing to another antivirus software solves it. But I have bought a year licence for Bitdefender Total Security 2013 and I want to use it. The game runs fine but after ~5 min playing single-player the FPS drops to 1 frame per second and after some time I get this message: "Ran out of virtual memory. To prevent this condition you must free up more space on your primary hard disk." Any ideas?