Hello,
This problem may causes by 3rd-party add-in, we can test mail merge in Safe Mode, you can run this command: winword.exe /safe.
- In Windows 7 or Windows Vista, click Start, click All Programs, click Accessories, and then select Run.
winword.exe /safe
Note: There is a space between "winword.exe" and "/safe".
If the problem does not appear in Word safe mode, you can disable add-ins troubleshoot this problem.
You can follow the steps "Disable the Startup folder add-ins" and "Delete the COM add-ins registry keys" in the KB article:
http://support.microsoft.com/kb/921541
Best Regards.
William Zhou
TechNet Community Support
