Hi Lamav,

there are actually many free radius available like TekRADIUS or FreeRADIUSetc. If you have any microsoft server that can be use Internet Authentication Service (IAS). IAS is the Microsoft implementation of  a Remote Authentication Dial-in User Service (RADIUS) server and proxy  in Windows Server 2003. As a RADIUS server, IAS performs centralized  connection authentication, authorization, and accounting for many types  of network access, including wireless and virtual private network (VPN)  connections. As a RADIUS proxy, IAS forwards authentication and  accounting messages to other RADIUS servers.

config IAS:-

http://technet.microsoft.com/en-us/library/cc758846%28WS.10%29.aspx

thanks,

Vinay