[graphviz] Why is pydot unable to find GraphViz's executables in Windows 8?

I tried adding PATH via Control Panel, but restarting the command prompt is also needed. The following also works for updating the path in a Notebook session without exiting:

import os     

os.environ["PATH"] += os.pathsep + 'C:/Program Files (x86)/Graphviz2.38/bin/'