If you are one those (un)happy people to still having to use DOS applications under Windows XP, then it is possible that your applications no longer run.
If you used high memory then this won't work any longer and you will receive out of memory messages from the DOS application.
The reason for this are a few recent microsoft updates for windows XP.
In more detail,
deinstall these 4 updates and restart the computer
You will then have back the high memory area for your DOS applications.
Why they deactivate high memory is not clear, but I think it's (realy, realy) time to replace your DOS application (and Windows XP at the same time) by a modern solution.