I tried your suggestion (thanks), it doesn't appear to be the problem. I am running an older version of NAV (7.0 Corporate, which has a never-ending license, updated with WindowsXP patch). It does not appear to have Office Plug-ins. Also, I tried to see if the popup box was behind the Word window, but it is not there when I iconize the window, and it still does not appear if I do the spell check with the Word window off the screen. Hmmm... I will try one more thing here
Found it! The popup is being displayed on the secondary monitor (an HDTV, infrequently used with the computer). I am using an nVidia GeForce4 MX 440 video adapter. Other programs send their popups to the primary monitor (like Excel 2003, for instance). I am searching through the video settings to see if there is something causing this - I'll post if I find a solution, please advise if you know a setting in Word or the video properties which affects this.