It seems many have had issues with advanced dof in metro 2033. The effect isn't even that great, just leave it off, + I don't think a hd6870 can even handle playable fps with adv. dof on very high settings at 1080p. Although updating drivers may fix the issue, don't install new ones over old ones, sweep them first.
Intel I7 3770K @ 4.5ghz, Cooler Master Hyper 212 Evo, Gigabyte Z77X-UD3H, Gigabyte HD7970 ghz edition, 4x4gb Kingston HyperX Beast 2133mhz, Seasonic Platinum 860, Samsung 840 Pro 256gb, OCZ Vertex 4 128gb, WD Black 1TB, WD Green 3TB+1.5TB, ASUS Xonar Essence ST, ASUS VE278Q, Windows 3.11