Field overflow causes client to lockup

We are running only one Windows 2000 Pro desktop. When this user is entering data in text fields he recieves no end of field warning, and can apprantly continue typing but by the time he notices no characters are appearing on the screen he is locked up. The freeze up occurs only in Navision. He can close the app and return to other apps, but cannot do anything with Navision. This does not occur on any other system we are using, running Win 98 or XP. Any suggestions??