To solve this problem,when you install graphviz2.38 successfully, then add your PATH variable to system path.Under System Variables you can click on Path and then clicked Edit and added ;C:\Program Files (x86)\Graphviz2.38\bin to the end of the string and saved.After that,restart your pythonIDE like spyper,then it works well.
Don't forget to close Spyder and then restart.