Pidgin, a multi-protocol instant messaging client (which supports, among others, Windows Live Messenger, Yahoo! Messenger, AIM and ICQ) that can also serve as a basic IRC client. It can be downloaded for Windows for free here.
To add an IRC server to Pidgin, click on the "Accounts" button, and then on "Manage Accounts".
Once the "account" window pops-up, click on the "Add" button.
Once the "Add Account" window pops-up, you will see the "Basic" tab. Click on the drop-down box next to "Protocol" and chose "IRC". Enter your ADISC username in the "Username" field, "adisc.org" in the password field and your ADISC password in the "Password" field. Make sure the "Remember password" box is checked. Once everything is in order, click on the "Advanced" tab.
The "Port" field should have "6667" by default - this is the correct setting. It is recommended that you enter something in both the "Username" and "Real Name" field to protect your privacy (as Pidgin may end up using a username associated with another IM client and the name associated with your computer's account) - what it is specifically really doesn't matter as long as you keep in mind that both fields are publicly visible.
For the "Encodings" field, either do not modify it or fill in "UTF-8".
The "Use SSL" checkbox should not be checked.
Once all the settings are properly filled, click on the "Save" button.
Your IRC session should open automatically every time you open Pidgin, and should look something like this: