Ubuntu n'est pas détécté

Bonjour,

je possède un PC portable tatoué, sur lequel j'ai installé ubuntu via la alternate version tout en conservant Windows XP. Pour cela j'ai organisé mon disque dur, de la façon suivante :

/dev/sda1 (fat32) ou est installé le système de restauration (MBR)

/dev/sda2 (fat32) ou sont installés les programmes tournant sur windows

/dev/sda3 (ext3) la racine

/dev/sda4 partition extended ou il y a :
- /dev/sda5 (fat32) partition qui est censé me servir pour stocker mes fichiers sur les 2 systemes
- /dev/sda6/ (ext3), partition /home
- /dev/sda7/ la swap

Après installation et en ayant bien fait attention d'installer GRUB sur /dev/sda3, le système au démarrage, ne semble pas reconnaître Ubuntu, et me lance automatiquement windows alros que j'aimerais avoir le choix au démarrage entre les deux systèmes via GRUB. Je me suis tourné vers le fichier GRUB (media/disk-2/boot/grub/menu.lst) en y apportant quelques modifications, Pourtant rien. Ubuntu n'est toujours pas détecté.

Je vous communique mon fichier GRUB :

# 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 change this entry to 'saved' or your
# array will desync and will not let you boot your system.
default 1

## 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=/dev/sda3 ro

## default grub root device
## e.g. groot=(hd0,0)
# groot=(hd0,2)

## 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

## 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

## 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

## ## End Default Options ##

title Ubuntu, kernel 2.6.15-26-386
root (hd0,2)
kernel /boot/vmlinuz-2.6.15-26-386 root=/dev/sda3 ro quiet splash
initrd /boot/initrd.img-2.6.15-26-386
savedefault
boot

title Ubuntu, kernel 2.6.15-26-386 (recovery mode)
root (hd0,2)
kernel /boot/vmlinuz-2.6.15-26-386 root=/dev/sda3 ro single
initrd /boot/initrd.img-2.6.15-26-386
boot

title Ubuntu, memtest86+
root (hd0,2)
kernel /boot/memtest86+.bin
boot

### 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/sda2
title Windows XP Media Center Edition
root (hd0,1)
savedefault
chainloader +1


# This entry automatically added by the Debian installer for a non-linux OS
# on /dev/sda1
title Windows NT/2000/XP
root (hd0,0)
savedefault
chainloader +1

Pouvez-vous me communiquer une solution s'il vous plaît.