DOS must be installed on the boot partition (C:) which must be formated as the FAT file system. Before doing anything, generate a new ERD (Emergency Repair Disk) by running rdisk.exe /s. You will also need your setup floppies. If you lost them, run Winnt32 /ox from the CD-ROM.
After installing DOS, the Windows NT boot loader will be disabled. To enable the boot loader, boot from the NT setup floppies and choose Repair (only repair the boot records). After the repair, boot Windows NT. From a command prompt, type
attrib -r -s -h c:\boot.ini
Edit c:\boot.ini and add c:\="DOS" (or whatever text you want) to the end of the operating system section. Then type
attrib +r +s +h c:\boot.ini
On you next boot, you will see both the NT and DOS options on the boot menu.
If you want to install Windows 95 also, boot to DOS and install Windows 95. Windows 95 is NT-aware and will not destroy the boot loader if installed from a dual boot DOS session.