xmspam is a site-wide XMail filter which calls spamc (SpamAssassin client) and adds the X-Spam headers in the message file.

UPDATE: this filter has been superseded by this howto (currently only in French, sorry).

xmspam will just add the headers - no mail will be dropped.
This is a site-wide implementation. If you need more features and you like C#, the code is very simple and easily customizable !
If you don't like C#, please drop me a line, maybe I'll have a solution...
I haven't tested xmspam under Windows, but I suppose it should work.

xmspam is licensed under GNU GPL v.2 and is downloadable here.