Try the following:
1. Go to Start Menu > All Programs > and right click on the Internet Explorer icon.
2. Choose "Run as Administrator" or â€śRun As ...", depending on your operating system. (If "Run As": Select the drop down to "Administrator" and then type in the administrator password, if needed.) Confirm your selection, as needed.
3. This should open up Internet Explorer, running as the Administrator.
4. Type "C:" in the address bar at the top, and Windows Explorer should open up (but you are in the Administrator account). [Note: If you have an icon on your desktop for Windows Explorer, you can just right-click it and open as Administrator -- same result]
5. Look for the Control Panel link on the left, and click it.
6. Try to Add / Remove the Yahoo Toolbar ... but this time you are in the Administrator account, so it may work for you.