You sure you obtained the entrypoint for the extension function?
Or, as you say, maybe your driver doesn’t support this extension. Just do a glGetString(GL_EXTENSIONS); and see the result. If the string doesn’t contain GL_MULTITEXTURE_ARB (or something similar), your driver doesn’t support this.
And running GLSetup will only help if there are drivers for your card that does support this extension. And by the way, does GLSetup work for Win2k now? Think it only works for Win9*, but not sure (this was the case a while ago at least, but I have heard some people talking about Win2k support).
This more likely sounds like a misunderstanding. You have to obtain the entry points for opengl-extension functions (and multitexture ARB is an extension) with wglGetProcAdress(“name”). You can look wether your opengl implementation supports mutlitexturing when you get the GL_EXTENSION string. It should contain something like GL_EXT_MULTITEXTURE_ARB. If it is supported, you can get the entry-points of the needed function. That is quite a bunch of work though…
Well, if your driver doesn’t support this extension, the result should be pretty obvious. You can’t use it. And yes, obtaining the entrypoint yourself is the only (?) way of doing multitexturing on Windows (on OpenGL 1.1 and below at least).