I downloaded Graphviz 2.38 MSI version and installed under folder C:\\Python34, then I run pip install Graphviz, everything went well.
Graphviz 2.38
C:\\Python34
pip install Graphviz
Try using:
conda install python-graphviz
The graphviz executable sit on a different path from your conda directory, if you use pip install graphviz.
pip install graphviz