synced_folders/nfs: unavailable on Windows

lib/vagrant/action/builtin/mixin_synced_folders.rb

@@ -19,7 +19,7 @@ module Vagrant
             ordered << [priority, key, impl]
           end
 
-          # Order the plugins by priority
+          # Order the plugins by priority. Higher is tries before lower.
           ordered = ordered.sort { |a, b| b[0] <=> a[0] }
 
           # Find the proper implementation

plugins/synced_folders/nfs/synced_folder.rb

@@ -25,6 +25,11 @@ module VagrantPlugins
       end
 
       def usable?(machine, raise_error=false)
+        # We don't currently support NFS on Windows
+        if Vagrant::Util::Platform.windows?
+          return false
+        end
+
         # NFS is always available
         true
       end