Installation from source is the best way to install any software, including SpamAssassin. Tools like CPAN and CPANPLUS come in really handy when installing SpamAssassin on a machine. Installation on Windows can be achieved by using Cygwin or ActivePerl.
Installation prerequisites for SpamAssassin are:
SpamAssassin can also be installed in the RPM and Debian formats. This chapter covered installation on RedHat, Debian, and Gentoo Linux systems as well as on Windows using Cygwin.