Finding a suitable PBX for Windows is not so easy. Most of the free versions are actually free only with a few users. The exception was FreeSWITCH which is open source and runs on windows, however it is not so easy to use for the average users (no GUI, regexp rules for routing).

With the Compact Softswitch project our intention is to fill this gap on Windows by a solution which is both free and easy to use.

Not sure what the status is now, but the https://sipsorcery.codeplex.com/ project ("C# SIP protocol stack") withered after they implemented their admin UI in Silverlight.

After a bit of digging all the pieces required for self-hosting appear to still be there.

Not for Windows but XiVO is a nice Linux user friendly PBX appliance: http://www.xivo.io