If I correctly understood the question then H.323 client is included in Windows 2000 and you can use it via dialer.
There is free H.323 library with examples of H.323 s/w phones. You can download it from http://www.openh323.org.
We ended up buying a H323 sofphone license from Cirilium, GW and a GK also. It goes by licences.
A bit expensive but it runs ok.
There is another one that I tried and works fine from H323 EyePPhone Desktop.