Graphviz ruby on windows

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
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,

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