For windows have a look at
http://www.advancedispbilling.com/
http://advancedradius.com/

I would suggest that you look to using Linux/BSD as the OS on which to base a wisp - aside from being a much better server OS than windows, there is heaps of good isp and network software available, much of it open/free.
Have a look at
http://www.sisd.com/freeside/
http://www.freeradius.org/
http://nocat.net/
http://www.hpi.net/whitepapers/warta/

As well as authentication and billing you will need to manage bandwidth and priority, both of which are much easier using Linux.