Zero on keyboard

I have a strange problem with the keyboards at my job. When I open a DOS window, the zero key on the number pad does not want to work. The zero key on above the alpha characters works fine. On Windows the zero key on the number pad works well also. I tried changing drivers, changing keyboards, but that does not work. This is a problem that is happening on several computers with Windows 98 installed on them. I did not use an imaging software on all of them so a corrupted driver in an image file is not the issue. Please help.