To use noVNC with existing UltraVNC server you need to do some simple steps. When you launch js vnc client, it will connect to websockify - websockect to tcp proxy, which will connect to your vnc server. Here are these steps:

Step 1. Configure vnc server
You should install and configure UltraVNC server on your destination machine. For example: install vnc server on Windows 7 machine with IP: 192.168.1.100 and configure them to listen on 5555 port.

Now you can connect to 192.168.1.100:5555 using UltraVNC client. If your connection is successfully established you can go to the next step.

Step 2. Configuring environment for websockify

Download and install python. I have tested websockify with python 3.2.5

After installing python you should get latest version of noVNC.

Extract zip archive to C:\noVNC.

Step 3. Launching websockify

Open cmd.exe and run:

C:\Python32\python.exe C:\noVNC\utils\websockify 7777 192.168.1.100:5555

Now open C:\noVNC\vnc.html in browser which supports websocket. Enter 127.0.0.1 in Host and 7777 in port, press Connect.

You will get Server disconnected (code: 1006) error and in cmd you will see the following error:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Python32\lib\multiprocessing\forking.py", line 374, in main
    prepare(preparation_data)
  File "C:\Python32\lib\multiprocessing\forking.py", line 497, in prepare
    '__parents_main__', file, path_name, etc
  File "C:\noVNC\utils\websockify.py", line 1, in <module> websockify
NameError: name 'websockify' is not defined

To fix this error you should delete websockify.py from C:\noVNC\utils directory and copy C:\noVNC\utils\websockify to C:\noVNC\utils\websockify.py. So you will have two identical files in C:\noVNC\utils\ : websockify and websockify.py

Now you could relaunch websockify and connect to your server from noVNC client.