Dave, I am still using XP Pro and loving it, but using IE8. You can disable it, if you look in the top RH corner of IE and there you may see Bing and a magnifying glass with a down button.
If this there then click on the down button, and it will display the search engines that are loaded on your PC. Now select "Manage Search Providers" ( left click) and rright button click on the offending program. It then gives you a dialog bax to disable the program, do so (left button click).
While Bing is highlighted, look further down the page and you will see a check box to "Prevent programs from suggesting changes to my default search provider" do that too. Last thing on that page is to select the search engine that you do want as default (say Yahoo! Search) up the top then left button click on it; then go down to the bottom of that page and you should see another button that says "Set as default" click on that; then click on close.
Hope that helps and no confusing to you...........