Note: The enthought package includes numpy, scipy, Mayavi and TraitsUI (they offer a free license for academic users; there is also a ‘free’ version of the package on the website, but unfortunately it does not have advanced components such as Mayavi and TraitsUI). This package also contains ipython, matplotlib and many other useful tools. For pyfits, you can either download it separately, or simply install the astropy package. Therefore, the minimum requirement is two installations: the enthought package and pyfits/astropy.

Update on May 19, 2013: Just found that the enthought package has pyfits in it! So all you need to do is download and install it. However, since astropy is a promising project that plans to incorporate all astronomical modules into one (such as ATpy and pywcs), it is definitely worth trying if you want to explore more.

Update on Apr 28, 2014: Now the enthought package is call 'Canopy'. They still provide a free license for academic users. Canopy includes astropy. Another option is the Anaconda package which also includes all the required modules (mayavi, traitsUI, astropy, etc.). (And it's free, too.) Please install either one, then everything should work.