1-2-all BroadCast Email, its php, so it could run on windows as well, but I run it under freebsd.
It works great and I cannot recommend it enough. I've been using it for almost a year now to send out a weekly affiliate newsletter to 750 or so affiliates at a time. It takes about 5 minutes to send out that many.
I havent even explored all of the features, but it will import an affiliate database from CCBill nicely, it has variable data insertions and supports clicktracking in html email, plus it can send out both html and ascii email so those that dont use graphical email can still read it.
I seriously love this app for newsletters.. If you have questions about it drop me a line, I'm quite familliar with it.