Daemon option is not supported on Windows.

lib/guard/rails/runner.rb

@@ -42,7 +42,7 @@ module Guard
         '--pid', pid_file
       ]
 
-      rails_options << '-d' if options[:daemon]
+      rails_options << '-d' if options[:daemon] && !windows?
       rails_options << '-u' if options[:debugger]
       rails_options << options[:server] if options[:server]
 

spec/lib/guard/rails/runner_spec.rb

@@ -40,7 +40,7 @@ describe Guard::RailsRunner do
       end
     end
 
-    context 'daemon' do
+    context 'daemon', :unless => RUBY_PLATFORM =~ /mswin|msys|mingw/ do
       let(:options) { default_options.merge(:daemon => true) }
 
       it "should have a daemon switch" do