Need help

Sorry bout this, I know almost nothing about OpenGL and this may be a very simple question but pleases help. Whenever I try to play a OpenGL game (at least I think there OpenGL) like Medal Of Honor, Jedi Knight2 ect I get this message at the end:
GLW_StartOpenGL() - could not load OpenGL subsystem

I cant play my games, if you can try and make the explanation as easy to understand as possible that would be cool. I have windows XP and NVIDA Geforce2 graphics card dont know wether that matters, I didnt always get this it used to work great but I used to have Windows 98 I dont know wether it started doing this after the switch or not, thanx.