sybok
The machine boots to the Windows boot manager which allows me to choose whether to boot up Windows 7 or FreeDOS.
0) Resize partitions to make room for antiX and create a new partition or new partitions.
Backup data before and beware, any error during this operation may lead to loss of data (my live system once froze and messed-up my Windows 10 partition in progress of resizing).
This can be done from the live USB.
You have already done that; you may consider to have a separate root ‘/’ and ‘/home’ partitions (it has its advantages – reinstall while preserving /home – and disadvantages – if one of the partition sizes is later found as too restricting)
1) Install antiX to the dedicated partition(s) with the grub installed to MBR on the drive.
Reboot once done.
2) If the other OSes are not detected, proceed as follows:
Modify /etc/default/grub so that it contains line ‘GRUB_DISABLE_OS_PROBER=false’ (possibly commented out or the value might be set to ‘true’).
Then run ‘sudo update-grub’ which recreates the grub and it should detect the other OSes, i.e. Windows and FreeDOS.
3) If that fails, my advice was not good enough 🙁 and you may give a boot-repair tool in antiX live USB or some specialised Linux distribution a try.