Sad but true: It is not likely that MiKTeX 2.7 works on legacy Windows systems (9x/Me/NT). It seems that the new build system, Visual Studio 2008, does not support legacy targets.

I am planning to make available a beta version of MiKTeX 2.7 in September. This will contain XeTeX 0.996. The 2.7 status page will be updated soon.