xmspam is a site-wide XMail filter which calls spamc (SpamAssassin client) and adds the X-Spam headers in the message file.
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.
I haven't tested xmspam under Windows, but I suppose it should work.
xmspam is licensed under GNU GPL v.2