I\'ve read through the FAQ for Jython and this post Jython and python modules but am not sure how I can determine if a module is written purely in C or Python.
Did you add C:\Python26\Lib\site-packages\nltk to sys.path as stated in your question? It should really be C:\Python26\Lib\site-packages, which is the directory containing nltk. If you don't want to add the entire site-packages, try moving the nltk folder to another folder.