Salem

Hello, does anyone know how to fix this?:

My graphic card is ATI Radeon HD 5700 and using windows 7.

Cheers.

Check this: viewtopic.php?f=10&t=96
If it doesn't help then try updating graphics card drivers

"All ATI users will need to make sure to turn off the OpenGL setting "Triple Buffering". It's not supported and will cause the client to crash after character select."

Can anyone explain me how to do that?

Well, I don't have ATI card, but here's where you can find the answer:
http://lmgtfy.com/?q=how+to+turn+off+op ... ng+%2B+ati

Which should lead you to here, for example: http://forums.steampowered.com/forums/s ... ?t=1887033

if you have an ATI card you should have Catalyst Control Center (CCC) installed for managing your graphics card and drivers. In the CCC program, it is under the gaming menu.