Ruby Graphviz Windows - can you help me?

I don’t manage to work this file on ruby:

require ‘graphviz’

Create a new graph

g = :G, :type => :digraph )

Create two nodes

hello = g.add_nodes( “Hello” )
world = g.add_nodes( “World” )

Create an edge between the two nodes

g.add_edges( hello, world )

Generate output image

g.output( :png => “hello_world.png” )

coz it gives me an error:

"C:\Ruby193\rubywork>ruby helloworld.rb
C:/Ruby193/lib/ruby/site_ruby/1.9.1/graphviz.rb:544:in output': GraphViz not in stalled or dot not in PATH. Install GraphViz or use the 'path' option (StandardE rror) from helloworld.rb:14:in


I installed Graphviz program, but…I don’t know why it doesn’t work.

Can you help me?

Thank you so much,