Win95/DOS drivers

I am running a credit card settlement program. It uses an external modem supplied by Visanet called a posport. We have used this posport on Win95, 98, and DOS. I have never had a problem or have I ever had to set up any special drivers. In onelocation there is a machine that was ordered with NT on it by the local IT guy. To install our credit card server on it he wiped the drive and put a basic win95 set up on it. He call it a a vanilla setup.
The only way we can get the POSPORT to work is to 1. shut down and restart in ms dos mode. Then type exit and have windows restart. It will not work in windows on this machine and it won’t work in dos. Only the shut down/exit gets it to work?

Thanks a lot
John Bennie