Windows 7 Counterstrike: Source

Hello there,
I got a bit of a problem here.
Just installed the newest version of Windows 7 beta from MS' homepage.
And I installed Steam and CS:S, but when i enter CS:S the screen ain't fullscreen. It haves a 5cm space in each side, where its just black. Tried every single resolution, and none of em' completes the full screen. Only when i'm setting it to Windowed mode its full screen, but then the text and graphics are totally unclear.
I hope you can help me
Thanks for reading

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