However, i read that Direct 3D 11 is just an extension Direct 3D 10 if i recall correctly, if not then i apologize if i am mistaken.
Nope. D3D11 is an entirely new API. It's one that looks a _lot_ like D3D10, but it's different. Different headers, different class names, etc. (mostly).
One nice thing D3D10+ has is the idea of feature levels. You can use D3D11 but configure it to only use D3D10 or even D3D9 hardware capabilities. This means that you can easily support a wide range of hardware with only the newest API version and without nearly the same number of contortions that OpenGL requires.
Windows XP only supports D3D9. Windows 7 shipped with D3D 11.0. The only OS that supports D3D10 but not D3D11 is unpatched Vista (I believe SP1 added it). All versions of Windows support OpenGL up to the latest version (though the specific version, extensions, and bugs supported vary by driver vendor).
The only reason to use D3D 9 at all anymore is if you plan to support XP, only need D3D 9 hardware features, and don't ever plan to port to non-Windows OSes via GL. The only reason to use D3D 10 is if you plan to support the ~5 gamers using Vista RTM. GL is an option, though (IMHO) not a great one if you don't need the extra platform support.
There are even a few nice D3D 11.1 features that are very handy for debugging that you can conditionally enable if available. Windows 7 doesn't have "full" 11.1 but it does have a Platform Update available that enables parts of it, primarily those you'd use for development and testing.