If you look at the BeOS installation CD from within WindowsNT, you’ll find a folder called „NT.“ Within it is a file called addbeos.exe.
Open an NT shell, change directories to your CD-ROM drive’s NT folder, and type addbeos. This will make changes to your boot.ini and drop a file called BeOS.bs in the root of your boot drive. The next time you boot, BeOS will appear on your NT boot menu.
If this does not work for you for some reason, Mark Yen (bemyen@bemail.org) adds the following information:

Requirements
• A working Windows NT (I had 2000 Pro)
• A working Win9x or DOS dualbooting under NT’s boot menu (I had 98SE),
• loadbeos.com and related files (e.g., from PE)
• Not currently triple-booting NT, 9x, and DOS

Procedure
1. Back up everything under C:
2. Copy the following files:
   If you have Win9x:
   • BOOTSECT.DOS to BOOTSECT.W40
   • IO.SYS to IO.DOS
   • MSDOS.SYS to MSDOS.DOS

   If you have DOS:

   • BOOTSECT.DOS to BOOTSECT.W40
   • IO.SYS to WINBOOT.SYS
   • MSDOS.SYS to MSDOS.W40
3. Edit Boot.ini in C: to have something like:
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)WINNT="Microsoft Windows 2000 Professional"
/fastdetect
C:BOOTSECT.W40="Microsoft Windows 98" /WIN95
C:BOOTSECT.DOS="BeOS Personal Edition" /WIN95DOS

   The first 2 OSes were put in by NT on install for me (to dualboot Win98 and NT).
   Be sure to have the /WIN95 and /WIN95DOS switches (even for another Win9x).

4. Make AUTOEXEC.DOS (or AUTOEXEC.W40 if you had DOS
   instead) to something like:
   
@C:
@CDBEOS
@LOADBEOS


5. Reboot and hope.

If you started with Win9x, you might want to edit MSDOS.DOS to
include Logo=0 under [Options] to turn off the Win9x
logo when booting BeOS.