SpamAssassin for Win32 Download
SpamAssassin is one of the best spam filters and it is also free. However, it is written on Perl and runs usually on corporate UNIX servers.
The early versions of SpamAssassin did excellent job killing spam, but they just could not run on Win32 systems. The first successful attempt to port SpamAssassin to Microsoft Windows family was made by Dan McDonald, who wrote a pop3proxy script which was acting as a proxy between the mail client and the POP3 server.