DOS apps running under Win2000 Help?

We still have a program at my company that basically runs the whole business that was written in DOS. Since I installed Win2000 on some of the new/old PC's the DOS program works so slow that everyone is complaining. I have tried just about every trick I know to make it faster but to no avail. This program only uses the 640 conventional memory area. It doesn't matter if the PC has 128K mem or 256K, it still runs slow. On a Win95/98 it works fine. If you open more then one application of this program, it's totally unusable. Can anyone out there help me?