This error is caused by a Windows security update published December 2009, affecting Windows XP, Windows 2000 and Windows Server 2003. You can resolve the issue by disabling the converter through the registry. Open regedit and delete the registry keys at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Shared Tools\Text Converters\Import\MSWord6.wpc. Restart Word and the error should disappear.

You can find more information in this article. It also provies a script that will perform the registry fix for you. The additional steps mentioned in the update to that article have not been tested, as unregistering the converter has solved the problem in every instance in which we've encountered it.

Note that this issue only affects Word 2007 SSRS templates. Word 2003 templates are not affected. Also, Excel 2007 is not affected by this issue.