Highlighted Text / Popup menus in Internet Explorer / XP

My browser has some new behaviour that is very nasty.

Special keywords are being highlighted in yellow, and when you mouseover them it brings up a popup menu. For example, when the word "business" is found in the page, it makes it bold and gives it a yellow background. This occurs across all sites, with a range of keywords including "computer", "credit" and even drug names. The menu redirects to links like this:

http://yafoo.biz/search.cgi?acc=1326&q=Home Insurance

Ad-Aware and Spybot can't seem to find anything wrong, even with updates and running in Safe Mode (the problem occurs in Safe Mode - even with no internet connection enabled.)

When I view source, I find a bunch of style entries and javascript functions to create the floating menu.

<style>
.xxxxmenu
{visibility:hidden;background-color:#ECF8FF; etc etc

.xxxxkeyword
{background-color:#FFFF00;color:#000000;

function onmo(obj)
{
var objname = obj.getAttribute('id')+'_menu';
inMenu = true;
var spn = document.getElementById(objname);

The keyword is surrounded by a span:
<span id="BUSINESS_menu" class=xxxxmenu onmouseout="menuonmo(this);" onmouseover="menuonmover(this);">
<b>Category:</b><a href='http://yafoo.biz/search.cgi?acc=1326&q=Business' target='_blank'>Business</a>

How does this stuff get inserted into the page? How do I get rid of it???