the platform is windows

Hi!
I think OpenGL is a graphics API only (please correct me if I'm wrong).
DirectX & XNA also offers support for devices as input (mouse, keyboard, joystick, gamepad, etc.) as well as sound, music, etc.

I think DirectX will persist, and if you learn it, u will probably also know XNA, as this last one is rather similar, and a bit simplifyied (some kind of wrapper around DX).

If u plan to port ur games to XBox, XNA offers u all the tools. Also, XNA encapsulates some lower level stuff that nowadays we normally don't need to deal with (as window creation, registration, initialization & this boring stuff).

I'm not sure about how long will XNA live, but anyway, as it's very similar to DX, whether u choose one or the other u won't be wasting time nor knowledges if one of them dissapears!

Hope this helps!

thanks Synthesizer because I agree with you, DX is a better choice to learn a new technologie.