Windows machines rarely ship with OpenGL ... eh ?
All windows desktop OSes (XP and above) do ship with OpenGL - all-be-it using a software renderer (GDI generic implementation) until a proper graphics driver is installed. Windows server OS don't ; but they're not exactly the norm for web surfers.
If you mean rarely ship with a full OpenGL 2.0 implementation then you may be correct (as the generic software renderer is only 1.1 compliant). Even then though the basic OpenGL calls should work, just not post 1.1 extensions (which should degrade gracefully if coded correctly).
If the PC ships with proprietary drivers (which many do) then it will most likely have full hardware accellerated OpenGL (upto 3.x) ready to go out of the box...