1.)First, go to your desktop, right click,properties then go to settings tab. Click advanced and go to Monitor tab, find your frame rate. After that, go to Mount and Blade Warband launcher, go to configure and change the frame rate to the frame rate of your monitor. Start the game! If doesn't work, go to 2nd option.
2.)Click Start then right click My Computer and click Properties. Go to Advanced tab and click settings on Performance section. Go to advanced tab and click the change below. Click Custom size and put 3000 on initial size and 300 in maximum size. Click ok and Restart your computer.
3.) If none of the two worked for you, open Mount and Blade Warband launcher and click configure. In the video tab, change the render method to DirectX 7 and change the screen resolution to 800x600 16 bits. Make the shadow quality low. You don't need shadows in game.
-If none of these worked, you should buy a new graphics or video card.