If you want to override the setting that opens new windows when searching from within Internet Explorer, here's what you can do.
Select the text you would like to search for.
Right-click on the selection.
Hold down the shift key while clicking on the search engine you would like to use
Here are a couple of examples of what will happen.
If the Open searches in new browser windows option is enabled and you hold down the shift key while clicking on the search engine, the search will open in that same window.
If the Open searches in new browser windows option is not enabled and you hold down the shift key while clicking on the search engine, the search will open in a new window.