Quick question: have you tried the various Asterisk distributions (AsteriskNOW, Trixbox CE etc etc)? They also come bundled as software appliances that you can run inside a virtual machine (if your app really really has to be on Vista).

If you tried them, I'm curious what your reasons were for choosing Avaya. In a previous lifetime I implemented a voice-banking prototype using Trixbox (at that time I think it was called Asterisk@HOME). It worked very well for me, but I couldn't get any banks in Kenya to agree to a demo, so the system is collecting dust in some svn repository somewhere.