我的机器上安装3个操作系统：redhat,redflag,win2000.安装顺序为：win2000->redflag->redhat.所以实现引导多OS的是redhat的grub。
修改redhat的/boot/grub下面的配置文件：grub.conf
(ubuntu的配置文件为:/boot/grub/menu.lst)
具体内容形如：
# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE: You do not have a /boot partition. This means that
# all kernel and initrd paths are relative to /, eg.
# root (hd0,9)
# kernel /boot/vmlinuz-version ro root=/dev/hda10
# initrd /boot/initrd-version.img
#boot=/dev/hda
#default boot redflag, the index begins with 0
default=2
timeout=10
splashimage=(hd0,9)/boot/grub/splash.xpm.gz
title Fedora Core (2.6.5-1.358)
root (hd0,9)
kernel /boot/vmlinuz-2.6.5-1.358 ro root=LABEL=/ rhgb quiet
initrd /boot/initrd-2.6.5-1.358.img
title Win2000
rootnoverify (hd0,0)
chainloader +1
#following is from the grub.conf file of Red Flag Desktop version 4.1
title Red Flag Desktop (2.4.26-1)
root (hd0,8 )
kernel /boot/vmlinuz-2.4.26-1 ro root=LABEL=/1 vga=785 splash=silent
initrd /boot/initrd-2.4.26-1.img
title Red Flag Desktop (Console Mode)
root (hd0,8 )
kernel /boot/vmlinuz-2.4.26-1 ro root=LABEL=/1 3
initrd /boot/initrd-2.4.26-1.img