Disque /dev/hdb: 41.1 Go, 41110142976 octets
    255 têtes, 63 secteurs/piste, 4998 cylindres
    Unités = cylindres de 16065 * 512 = 8225280 octets
 
    Périphérique Amorce    Début         Fin      Blocs    Id  Système
    /dev/hdb1   *           1        2442    19615333+  83  Linux
    /dev/hdb2            2443        4997    20523037+   f  W95 Etendu (LBA)
    /dev/hdb5            2551        4997    19655496    7  HPFS/NTFS
    /dev/hdb6            2443        2550      867447   82  Linux swap / Solaris

# 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'.
default         0
 
## timeout sec
# Set a timeout, in SEC seconds, before automatically booting the default entry
# (normally the first entry defined).
timeout         3
 
## 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 specifiv 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=root=/dev/hdb1 ro
 
## default grub root device
## e.g. groot=(hd0,0)
# groot=(hd0,0)
 
## 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
 
## altoption boot targets option
## multiple altoptions lines are allowed
## e.g. altoptions=(extra menu suffix) extra boot options
##      altoptions=(recovery mode) single
# altoptions=(recovery mode) single
 
## nonaltoption boot targets option
## This option controls options to pass to only the
## primary kernel menu item.
## You can have ONLY one nonaltoptions line
# nonaltoptions=quiet splash
 
## 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
 
## ## End Default Options ##
 
title           Ubuntu, kernel 2.6.12-9-386
root            (hd0,0)
kernel          /boot/vmlinuz-2.6.12-9-386 root=/dev/hdb1 ro quiet splash
initrd          /boot/initrd.img-2.6.12-9-386
savedefault
boot
 
title           Ubuntu, kernel 2.6.12-9-386 (recovery mode)
root            (hd0,0)
kernel          /boot/vmlinuz-2.6.12-9-386 root=/dev/hdb1 ro single
initrd          /boot/initrd.img-2.6.12-9-386
boot
 
title           Ubuntu, memtest86+
root            (hd0,0)
kernel          /boot/memtest86+.bin
boot
 
title           Windows XP
root            (hd0,1)
makeactive
chainloader     +1
 
### END DEBIAN AUTOMAGIC KERNELS LIST

Eug, g l'impression que ton fichier de conf ne reflete pas la situation (au niveau des root(hd0,0)..