Several similar posts have already been seen.
Can you please try to enable forced Nvidia Anti aliasing settings (through nvidia control panel, or with an additionnal software called Nhancer ( http://www.nhancer.com/ )
If that's not enough, you may try either :
1. running Extremes in full screen mode (by default it runs windowed)
2. or install GameBooster software, run in "Game Mode", and launch Extremes again (windowed) : AA should be ok.