Check the availability of conda pygraphviz for your system. The previous answer seems to be a package only available for linux and osx 64. Try it
conda install -c marufr pygraphviz=1.3.1
this version shows details that should be compatible with win32 and 64, as well as linux and osx. You can find more packages from any user who has typed
conda install -c binstar binstar
After that, for example, to find all the pygraphviz packages in the public code channels
binstar search -t conda pygraphviz
Finally, find your platform compatible and install from the command line, as usual.
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