How to default other with GRUB on boot screen ?

[bestofcomputer]

I have installed Fedora 7 (coz 8 not supported) on my old PC.
I could not specify which OS to start with GRUB during the installation because GRUB was already installed with a previous attempt to install F7 that did not work.
My second F7 install does not work neither : I cant boot fedora at all.
Now I want to change the default OS to boot with GRUB to OTHER and not to fedora coz it just dont boot, but I cant do that from a shell coz I cant boot fedora.
How to change the default OS from the boot screen of GRUB, coz I cant go to a full shell ?
Or do I have to use floppy or rescue CD ?
How ?
Thanks.

[CD-RW]

This works using the FC6 first installation CD or the FC6 rescue CD.

To re-install Grub to a non-bootable Fedora Box:

A
 1. Boot off the 1st Fedora CD or use the Fedora Rescue CD
 2. At the boot prompt type 'linux rescue'
 3. The rescue mode will find the fedora installation
 4. Type 'chroot /mnt/sysimage'
 5. Type 'grub-install /dev/hda' (to install Grub to the MBR of the 1st HD)
  (all commands without quotes)

Having re-installed GRUB to the MBR by following the instructions above,
I then need to do:

B
grub> find /boot/grub/stage1
 (hd0,12)
grub> root (hd0,12)
grub> kernel /boot/vmlinuz-2.6.18-1.2798.fc6 ro root=/dev/hda13
grub> initrd /boot/initrd-2.6.18-1.2798.fc6.img
grub> boot

Obviously, /boot/vmlinuz-2.6.18-1.2798.fc6 & initrd /boot/initrd-2.6.18-1.2798.fc6.img have to exist in the boot directory of the root partition for this to work.

You might get away with section B, without having to re-install GRUB as per section A. If you can do part A, that will allow you to access /boot/grub under /mnt/sysimage. You should be able to then edit your /boot/grub/grub.conf file.
The default OS to boot is set with:

# 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 (hd0,8)
# kernel /boot/vmlinuz-version ro root=/dev/hda9
# initrd /boot/initrd-version.img
#boot=/dev/hda
default=0

default=0  <---- will boot the first entry in the grub.conf file 
    ^
either change this to the entry to boot, or move the entry you want to boot first in grub.conf.

timeout=5
splashimage=(hd0,8)/boot/grub/splash.xpm.gz
hiddenmenu

title Fedora Core (2.6.22.9-61.fc6)
root (hd0,8)
kernel /boot/vmlinuz-2.6.22.9-61.fc6 ro root=/dev/hda9
initrd /boot/initrd-2.6.22.9-61.fc6.img

title Fedora Core (2.6.22.7-57.fc6)
root (hd0,8)
kernel /boot/vmlinuz-2.6.22.7-57.fc6 ro root=/dev/hda9
initrd /boot/initrd-2.6.22.7-57.fc6.img

title Fedora Core (2.6.20-1.3002.fc6xen)
root (hd0,8)
kernel /boot/xen.gz-2.6.20-1.3002.fc6
module /boot/vmlinuz-2.6.20-1.3002.fc6xen ro root=/dev/hda9
module /boot/initrd-2.6.20-1.3002.fc6xen.img

See 'pinfo grub' for more details.