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:
Perl
A C compiler
An Internet connection
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.