legecy DOS application

we have a network with a few PC’s and a novell serever we use a Legecy accounting application that runs under Dos, we just bought a few new Pc’s with win XP on them (the old ones were 9x) now when we run the application everything is fine except thefact that when we run in full screen the program just uses the top half part of the screen and the bottom is empty and when its not in full screen the windows is very small to work on. I need some help to tweek the system
Thanks