Bonjour à tous, 
Je vous remercie pour votre aide.
J’avais réinstallé Debian avant d’avoir pris connaissance de vos messages.
Cette fois, je vous le confirme j’ai bien indiqué (hd1,2) lors de l’installation du Grub pour Debian. Donc, plus de doute de ce côté là. C’est fait on en parle plus.
A partir de là, je pensais (dans ma p’tite tête 
) que ce fameux chainloader se faisait automatiquement puisque je venais de donner l’information, lors de l’installation, que le Grub de la 2ième distribution ne devait pas écraser la 1ère ?! Mais ce n’est pas le cas …
Donc sur vos conseils, je suis allé modifier le Grub d’Ubuntu pour lui indiquer qu’il devait désormais accueillir un nouvel arrivant en plus d’XP.
A ce propos, je trouve bizarre de laisser “chainloader+1” alors qu’il s’agit du +2, non ?
Voici le Grub en question :
[code]# menu.lst - See: grub(8), info grub, update-grub(8)
grub-install(8), grub-floppy(8),
grub-md5-crypt, /usr/share/doc/grub
and /usr/share/doc/grub-doc/.
default num
Set the default entry to the entry number NUM. Numbering starts from 0, and
the entry number 0 is the default if the command is not used.
You can specify ‘saved’ instead of a number. In this case, the default entry
is the entry saved with the command ‘savedefault’.
WARNING: If you are using dmraid do not use ‘savedefault’ or your
array will desync and will not let you boot your system.
default 0
timeout sec
Set a timeout, in SEC seconds, before automatically booting the default entry
(normally the first entry defined).
timeout 10
hiddenmenu
Hides the menu by default (press ESC to see the menu)
#hiddenmenu
Pretty colours
#color cyan/blue white/blue
password [’–md5’] passwd
If used in the first section of a menu file, disable all interactive editing
control (menu entry editor and command-line) and entries protected by the
command ‘lock’
e.g. password topsecret
password --md5 $1$gLhU0/$aW78kHK1QfV3P2b2znUoe/
password topsecret
examples
title Windows 95/98/NT/2000
root (hd0,0)
makeactive
chainloader +1
title Linux
root (hd0,1)
kernel /vmlinuz root=/dev/hda2 ro
Put static boot stanzas before and/or after AUTOMAGIC KERNEL LIST
BEGIN AUTOMAGIC KERNELS LIST
lines between the AUTOMAGIC KERNELS LIST markers will be modified
by the debian update-grub script except for the default options below
DO NOT UNCOMMENT THEM, Just edit them to your needs
## Start Default Options
default kernel options
default kernel options for automagic boot options
If you want special options for specific kernels use kopt_x_y_z
where x.y.z is kernel version. Minor versions can be omitted.
e.g. kopt=root=/dev/hda1 ro
kopt_2_6_8=root=/dev/hdc1 ro
kopt_2_6_8_2_686=root=/dev/hdc2 ro
kopt=root=UUID=4ca30c64-ef33-42e2-82df-b9646de2e5c1 ro
Setup crashdump menu entries
e.g. crashdump=1
crashdump=0
default grub root device
e.g. groot=(hd0,0)
groot=(hd1,0)
should update-grub create alternative automagic boot options
e.g. alternative=true
alternative=false
alternative=true
should update-grub lock alternative automagic boot options
e.g. lockalternative=true
lockalternative=false
lockalternative=false
additional options to use with the default boot option, but not with the
alternatives
e.g. defoptions=vga=791 resume=/dev/hda5
defoptions=quiet splash
should update-grub lock old automagic boot options
e.g. lockold=false
lockold=true
lockold=false
Xen hypervisor options to use with the default Xen boot option
xenhopt=
Xen Linux kernel options to use with the default Xen boot option
xenkopt=console=tty0
altoption boot targets option
multiple altoptions lines are allowed
e.g. altoptions=(extra menu suffix) extra boot options
altoptions=(recovery) single
altoptions=(recovery mode) single
controls how many kernels should be put into the menu.lst
only counts the first occurence of a kernel, not the
alternative kernel options
e.g. howmany=all
howmany=7
howmany=all
should update-grub create memtest86 boot option
e.g. memtest86=true
memtest86=false
memtest86=true
should update-grub adjust the value of the default booted system
can be true or false
updatedefaultentry=false
should update-grub add savedefault to the default options
can be true or false
savedefault=false
## End Default Options
title Ubuntu 8.04.1, kernel 2.6.24-19-generic
root (hd1,0)
kernel /boot/vmlinuz-2.6.24-19-generic root=UUID=4ca30c64-ef33-42e2-82df-b9646de2e5c1 ro quiet splash
initrd /boot/initrd.img-2.6.24-19-generic
quiet
title Ubuntu 8.04.1, kernel 2.6.24-19-generic (recovery mode)
root (hd1,0)
kernel /boot/vmlinuz-2.6.24-19-generic root=UUID=4ca30c64-ef33-42e2-82df-b9646de2e5c1 ro single
initrd /boot/initrd.img-2.6.24-19-generic
title Ubuntu 8.04.1, memtest86+
root (hd1,0)
kernel /boot/memtest86+.bin
quiet
END DEBIAN AUTOMAGIC KERNELS LIST
This is a divider, added to separate the menu items below from the Debian
ones.
title Other operating systems:
root
This entry automatically added by the Debian installer for a non-linux OS
on /dev/sda1
title Microsoft Windows XP dition familiale
root (hd0,0)
savedefault
makeactive
chainloader +1
GNU/Debian Linux installé sur /dev/sdb3
title Grub Debian
root (hd1,2)
chainloader +1
boot[/code]
Puis, j’ai fait un “update-grub”.
Debian est réellement installée (je vois les deux partitions (/ et /home) dans le poste de travail mais je ne sais pas comment y accéder par l’invite gdm ?
En tous les cas, lorsque j’allume le pc, Debian n’est pas présente dans le grub avec Ubuntu et XP.
Moi pas comprendre. 
Aïe !
)