I had the exact problem and the solution was really easy: Right-click on CS:S, choose Properties, click on Set Launch Options and write there "-dxlevel 81" (without "", you can also try 80, if 81 does not work).
I had the exact problem and the solution was really easy: Right-click on CS:S, choose Properties, click on Set Launch Options and write there "-dxlevel 81" (without "", you can also try 80, if 81 does not work).