Hi Troy,
I have seen this happen before when upgrading very old versions of Java. AddonChat requires Java 1.5 (or Java 5 as it is more commonly referred) which is now fairly old technology actually - about 5 years I think.
An offline (manual) installation is usually the way to go; making sure that all browser windows are closed before a user begins the offline installation. In the rare cases that this does not work we usually advise users to uninstall any version of Java they may have installed. Usually this can be done (in Windows) via the control panel add/remove programs utility.
People using virtually antique machines - I.e., Windows 95 running IE 4 or something may have an older Microsoft Virtual Machine (MVM) installed which can be disabled from their Advanced browser settings. We don't see much of this anymore though