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).