I haven't seen this before, but maybe something (or someone) modified your normal.dotm. You can open it and remove the box or simply delete the file if there are no valueable settings stored there (Macros for instance).
The template is located in the default template path of the user: %appdata%\Microsoft\Templates
If you assigned a workgroup template path, then the normal.dotm from there is used.
Edit: What you see is a text box (Insert|Text Box)