I haven't seen what's described here at mudcat, but several other sites do seem to think it's necessary to have a popup to explain "every difficult word." It's possible that you've visited a site that put the "find and define" utility on your machine, in which case it would look like an intentional add-in to your AV.

I haven't looked at any of these annoyances in any detail, but they appear to be nearly all incorporated via Java/JScripts. Microsoft lost the ability to incorporate their own version of Java in Windows a while back, so you have to get the Java applet from Sun now. There have been several Java exploits (malware vulnerabilities), with multiple patches and updated revisions recently.

Whether or not it's related to this nuisance, it would probably be a good idea to make sure to get rid of all older Java versions, especially in IE, and get only the most recent one in your IE, if you think Java is necessary at all.

Newest versions are claimed to be fully "backward compatible" so you no longer need to keep obsolete versions for obsolete websites (they say?), and if you don't have some version in your browser some sites will attempt to download an obsolete one without informing you. They shouldn't be able to do that, but it seems sometimes they can.

John