Using SpamAssassin on Windows
InstallingOnWindows: How can I install SpamAssassin on Windows?
SpamdOnWindows: How can I run SpamD on Windows?
SqlOnWindows: How can I use SQL for bayes and awl on Windows?
CompileOnWindows: How can I use sa-compile on Windows?
Michael Bell has written a HOW-TO on porting, installing, and using SpamAssassin on Win32 systems: http://www.openhandhome.com/howtosa.html (he points that SpamC will work for SpamAssassin 3.x) Also a configuration GUI http://www.openhandhome.com/saconf.html . For a while these links will not work, so you can find them on the Internet Wayback Machine site at: http://web.archive.org/web/20060214095852/www.openhandhome.com/howtosa310.html and http://web.archive.org/web/20060115220205/www.openhandhome.com/saconf.html.
Commercial Products
CommercialWindows: Commercial products you can buy and install for Windows.
Open-source Products
Jocafi Brazil contributed with a front-end to JSpamAssassin. It is a POP3 Proxy Server developed in Java (platform independent). It has a friendly GUI and acts as a communication interface between the user POP3 server and Apache SpamAssassin. It runs on all OS and works with all e-mail clients (Eudora, Pegasus Mail, Netscape, Thunderbird, Outlook Express, etc.).
Eric Smith wrote a simple VBScript to use SpamAssassin with Exchange 2000: http://www.spamblogging.com/archives/000014.html
Christopher Lewis wrote a much more robust VB ActiveX DLL that enhances Eric's original work, with the added benefit of actually placing the SpamAssassin mail headers in the Exchange mail message: http://www.christopherlewis.com/ExchangeSpamAssassin.htm
See also: WinSpamC
ExchangeSpamC is a Exchange 2000/2003 SMTP sink that combines Christopher Lewis' work with WinSpamC- http://sourceforge.net/projects/exchangespamc