If you’re talking about the Yahoo toolbar then there are a few ways to remove it, two of the easiest are as follows:
Using the toolbar itself:
Select UNINSTALL from the Toolbar Settings
Using the Control Panel in Windows:
Click the Windows START button. Select CONTROL PANEL from the SETTINGS menu. Select ADD/REMOVE PROGRAMS or PROGRAMS AND FEATURES (depending on the version) Select the YAHOO TOOLBAR entry. Click REMOVE or UNINSTALL.
Both methods will require a browser restart.
Regina, Saskatchewan; Canada
If it is the "Yahoo! Search" engine in IE8, I don't think it can be removed. You can find it under "tools" -> "manage add-ons" -> "Search Providers" but there isn't an option for uninstalling or even disabling it, unfortunately.