Complete crash to black screen with cursor (no the mouse kind)-Collection of common programming errors

I’m not sure whether it’s Steam that’s causing this, or just a co-incidence. When I load up Steam it tells me some stuff about my OpenGL and decreaced performance, so I press OK, then the client loads up for about 5 seconds then it just goes to a really low-res commandline where I cannot type anything.

I am using the AMD Proprietary Drivers 13.6 on an ATi Radeon HD 6870.

Thanks for your help guys.