XSPEC is part of the XANADU suite of data analysis programs. The files required for installing all or parts of the suite can be obtained via anonymous FTP at: heasarc.gsfc.nasa.gov in the directory software/xanadu
A short set of instructions for installing XSPEC and other XANADU programs can be found on the Web at the URL : http://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/index.html A LaTeX version (install_guide.tex) is available from the software/xanadu directory.
If you have any problems installing XSPEC, or have any questions, please send email to: xanprob@athena.gsfc.nasa.gov