Courtesy of Office MVP Beth Melton:

A "runtime" error means that there is an error in a macro or add-in that is
attempting to run. Typically these macros are created by a third-party and
are not part of the Word installation. (They could also be caused by a
malfunctioning macro virus as well)

Many third-party applications will create an add-in and place it in the
Office\Startup folder or use a COM add-in (DLL) to integrate their software
with Word. Another common add-in, the "Microsoft Works Suite Add-in for
Word" can only be found in your Add/Remove programs list.

Aside from the Works Suite Add-in, to determine if you have any add-ins you
can check for them using the following:

Go to Tools/Templates and Add-Ins and see if there are any global templates
or add-ins listed. If you find any then chances are they are located in
your Office\Startup folder or Word\Startup folder.

The location of the Word\Startup folder can be found under
Tools/Options/File Locations and the Office\Startup folder will be located
in the installation path for Office.

If you find more than one add-in move them one at a time until you
determine which one is causing the problem.

As of Word 2000, Word can have COM add-ins, those that are added in via the
Registry rather than the Startup folders. To check for COM add-ins add the
"COM Add-In" command to your toolbar.

- Right-click any toolbar and select Customize
- On the Commands tab, select the Tools category
- Locate COM Add-Ins on the right
- Drag/Drop to a location of your choice

For more information on COM Add-Ins, see this article:
http://www.mvps.org/word/FAQs/Customization/CheckIfAddinsInstalled.htm