Pygraphviz ValueError: program point not found in path

The problem is this: I'm on Windows7, and I installed the pygraphviz library and the Graphviz application (the Graphviz application is in "C: \ Program Files"). When I run my program in python, I get the following message:

g.draw(filename,prog='dot')
  File "C:\Python27\lib\site-packages\pygraphviz\agraph.py", line 1418, in draw
    data=self._run_prog(prog,args)
  File "C:\Python27\lib\site-packages\pygraphviz\agraph.py", line 1248, in _run_prog
    runprog=self._get_prog(prog)
  File "C:\Python27\lib\site-packages\pygraphviz\agraph.py", line 1236, in _get_prog
    raise ValueError("Program %s not found in path."%prog) 
ValueError: Program dot not found in path.

How can I solve this problem?