I had the exact same problem and i have no idea what causes it, i tried updating my direct x but that didnt work, updating graphics drivers, that didnt work either, personally id say its down to the drivers and direct x 10, just gotta wait for em to mature.
Its amazing how consoles always have crisp and amzing graphics and never have these major problems, oh wait these are pc's, yes thats right we have to buy exremely pricey items to get edges looking a little bit smoother.