In addition to links aforementioned I suggest that you take a look at
3DSpotLight's Full Guide to OS Dual Booting. Those combined should pretty much cover what's needed to boot between 9x & XP. XP performs the same way as Win2k, so if you see "Win2k" somewhere, just read it as XP.
After you've succeeded dual-booting with 9x & XP, boot from BeOS CD (if not using PE), install it and in the end install BootMan. Tick the partitions you want to choose from, which is default and how long it waits, and you're done.
If you want to add BeOS to your startup choices
after you've installed it, you'll need to do one of the following:
1 - Boot from BeOS boot floppy, run /boot/beos/bin/bootman
2 - Boot from BeOS CD, hit space during startup, select to boot from existing BeOS partition, run /boot/beos/bin/bootman
3 - Boot from BeOS CD, mount the BeOS partition with DriveSetup, hit CTRL+ALT+SHIFT+T in Installer screen to launch Terminal, run /boot/beos/bin/bootman and when it asks you to save the old MBR, replace
/boot with the BeOS partition's name you just mounted. Close Installer, don't install again
.