Okay,
I have a Nvidia graphic card, I don't know how it is for ATI's, but I will try to say what I did.
First, go to your System Panel and go to Deinstall Programs (I don't know what names these things have in english, but maybe I can help). There you should see something like this: NVIDIA Drivers and some other entrys about Sound Drivers or PhysX.
Now doubleclick on NVIDIA Drivers and choose "Deinstall Drivers". Then you have to reboot your computer and the resolution settings and so on will be horrible, but that's not a problem, you can change them fast.
Then go to: NVIDIA Drivers and choose your graphic card.
Download the driver, install it and enjoy teeworlds!
wapps.
burn the land and boil the sea, you can't take the sky from me.