The following steps allow you to install the latest Python 2.7 and nltk packages (tested under Windows) as of October 2014.
Download Anaconda for Python 2.7 (Anaconda is a distribution of Python pre-packaged with many useful packages.)
It includes Python 2.7.8 as of Oct 2014. Anaconda comes with nltk pre-installed.
If your Python installation does not include nltk, follow the instructions in nltk.
In Python, run:
>>> import nltk
>>> nltk.download()
In the window that opens, select "all" and download all corpora.