I would check a few things:
1 ) Defrag your gamecache
2 ) Verify Game Files
3 ) Verify and/or reinstall latest Direct X
4 ) Make sure you're not running something that forces a Direct X / D3D version or OpenGL forceware etc.
5 ) Maybe something in your config - I'd save the important shit like mouse and key binds elsewhere (a text file will do), delete your config, launch the game. This will auto-generate a new, standard config. Then exit the game, paste in the shit from your old config you saved and retry.
Holler, I've got more ideas.
EDIT: If you run CS:S using any custom skins, then I won't even touch that. It could be any one skin that could potentially cause this issue.