This might happen on a system where the legacy package was previously working, or may be triggered when moving the software to a different/new computer.
The DOS/4G extender was designed to help allow programs access memory above 640K in the pre-Windows era. Many different programs used the extender ranging from database applications and accounting systems to games. With the advent of Windows memory handling, its utility largely became moot and fortunately it is normally ignored when programs are run under more modern Windows 32-bit operating systems. Programs that were designed to use the extender however will require the presence of the DOS/4G executable (usually dos4g.exe or dos4gw.exe) or they will not load.
The issue relates to the Windows PATH that the DOS/4G extender was designed to handle. It has a limit of 250 characters (some sources indicate 255 or 260).
This issue is also unrelated to the specific operating system in use. Once the path environment variable exceeds the maximum, the DOS/4G extender error 2001 will occur. As programs are installed on, for example, an XP Pro PC, your path environment variable may start to become quite lengthy. Or on newer PC's with pre-installed programs, the PATH length may already be lengthy to start with. To see your current path, at a CMD line simply type:
PATH
If you want to determine its length, redirect the output to a text file, e.g.:
PATH > mypath.txt
Then retrieve the file, in this case mypath.txt, in NOTEPAD (uncheck Word wrap under Format and check on Status bar on view and you will then get a a line and column count in the status bar).
Another way to inspect your path would be under Advanced System Settings (how you get there depends on the operating system) and then Advanced and Environmental Variables. Changing/shortening your path here however may have adverse impacts when running other programs.
There are various potential solutions to the problem. The simplest perhaps is to simply edit the batch file (using a line editor such as NOTEPAD or if available EDIT) that starts the application (assuming one already exists) and simply add this line before the line that calls your application:
SET PATH=
(with nothing after the equal sign).
If your application does not use a batch file then create one with the SET PATH= statement followed by the name of your application (see your icon's properties). There are other solutions as well..
You may find that your application requires support from other folders and if so you may have to add a path or two to SET PATH= if that is the case:
SET PATH=C:\windows\system32;c:\someotherpath
Starting the application from the CMD prompt will initially help in troubleshooting errors.
More information:
Tenberry DOS/4G and DOS/4GW FAQ