If you already have a protected mode OS on your computer and you want to install DOS for executing HARTIK applications, you first have to create a new partition for it.
In order to perform this task, you have to create free space for the new partition: this can be done without loosing hard disk data using FIPS. More information on how to use FIPS can be found here. Once the free space is created, you can add the new partition using FDISK.
Now you can install the real-mode OS. If you have not a registered MSDOS version, you can download OpenDos from Caldera.
The installed OS can be booted using a boot manager, like LILO or GRUB.