To obtain and install Pidgin, follow the instructions at the Pidgin web site to download the Windows version. Installation is like any other program: just double-click on the file you downloaded and follow the on-screen directions.
Linux installation will be different depending on your distribution. You may be able to install Pidgin through your distribution's package management system (such as Yum, Synaptic, or apt-get), or you may need to follow the directions on the Pidgin web site.
When you start Pidgin, you will be presented with three windows. You can set up your account in the "Add Account" window. It should look like the picture below. Note that "NetID" should be replaced with your NetID. You must check "Force old SSL" and change the "Port:" field to 5223 for Pidgin to work properly.
Once you are configured, just click on the "Sign On" page in the "Login" window, and you will be prompted for your password. Use your Active Directory password. You will then be logged in, as below.
Depending on your role in the CS department, you may already have some contacts in your contact list. If not, you can add new contacts by choosing "Add Buddy" from the "Buddies" menu. Users of the CS department Jabber server are denoted as follows: NetID@jabber.cs.uiuc.edu.