http://ventrilo.com/download.php, download the windows client.
Install, then set it up...
User Name - New, type it in, if you want, you can type in a phonetic, and test it with "play"...that's what people will hear when you enter/leave rooms.
Server - New, name it whatever (I use "n00bfest")...
-IP/Hostname - vent2.clanservers.com
-Port - 4337
-Password - none anymore I think...
Bindings - skip, you can do some yourself if you want
Connect to the server, and then click "setup", the first tab is "voice", check the box called "Use Push-to-Talk Hotkey", and then go down to the "Hotkey" field below and click on it, and then press whatever key/mouse button you want. Now press "OK" and whenever you want to talk, hold down the button you chose while you speak. To navigate in between channels, double click on the name or someone in it. Some are passworded, some are public.