If your BeOS installation (or subsequent attempt to install a boot menu) goes horribly wrong for some reason, you may find that you can no longer boot into NT or 95. Reading the documentation thoroughly before you begin should help you to avoid this.
If things get screwed up anyway, you should be able to restore your original master boot record by typing fdisk /mbr at a DOS prompt. In most cases, this will return things to the way they were before. Note that, contrary to popular belief, fdisk /mbr does not restore a backup of the boot record — it simply writes the Microsoft boot record.
If you have a multi-OS machine with Be OS’s bootman and you have to re- install a Windows product, it’s gonna kill the MBR. Some folks will tell you to run FDISK /MBR to restore, but don’t bet on it. Better yet, boot from the Be OS CD and hold down the spacebar during boot. Select a different partition from which to boot and point it at the Be partition on the HD. Boot to Be and run bootman from within the Tracker (nice GUI!) by double-clicking on / boot/beos/bin/bootman. Re-install bootman!