A couple ideas come to mind as possible fixes for this. One is to download a freeware program called
CCleaner and run it. It's a disk cleanup and limited registry cleaning tool. Uncheck the box for Yahoo toolbar; you'll see it soon after starting the CCleaner installation. When using CCleaner, be sure to run the Issues scan and fix those items.
The other thing that comes to mind is to delete, or rename your normal.dot file. This is the template that stores your preference settings, customized toolbars, macros, etc. So, be aware that you will lose these by deleting the file. If you don't want to delete the file, then rename it; in either case (deleting or renaming), Word will create a new normal.dot file. Close Word when deleting or renaming the file. When Word is then started, it will create the new normal.dot file. The point of this is that the normal.dot file can become corrupted and cause problems.
You could run CCleaner, then see whether Word changes its behavior. Or, you could first try the normal.dot technique. It's a flip of the coin. CCleaner is a good tool to keep installed and run periodically for general system maintenance. So, if you don't have it, you might go ahead and get it and try it first.