I tried to install them using dpkg instead of the software center. The system did not crash but the result's not really better  : 

guitares-roiron@guitares:~/Bureau$ sudo dpkg -i *.deb
(Lecture de la base de données... 269877 fichiers et répertoires déjà installés.)
Préparation du remplacement de linux-headers-3.5.0-20 3.5.0-20.31+lp1087622v1 (en utilisant linux-headers-3.5.0-20_3.5.0-20.31+lp1087622v1_all.deb) ...
Dépaquetage de la mise à jour de linux-headers-3.5.0-20 ...
Préparation du remplacement de linux-headers-3.5.0-20-generic 3.5.0-20.31+lp1087622v1 (en utilisant linux-headers-3.5.0-20-generic_3.5.0-20.31+lp1087622v1_i386.deb) ...
Dépaquetage de la mise à jour de linux-headers-3.5.0-20-generic ...
Préparation du remplacement de linux-image-3.5.0-20-generic 3.5.0-20.31 (en utilisant linux-image-3.5.0-20-generic_3.5.0-20.31+lp1087622v1_i386.deb) ...
Done.
Dépaquetage de la mise à jour de linux-image-3.5.0-20-generic ...
Examining /etc/kernel/postrm.d .
run-parts: executing /etc/kernel/postrm.d/initramfs-tools 3.5.0-20-generic /boot/vmlinuz-3.5.0-20-generic
run-parts: executing /etc/kernel/postrm.d/zz-extlinux 3.5.0-20-generic /boot/vmlinuz-3.5.0-20-generic
P: Checking for EXTLINUX directory... found.
P: Writing config for /boot/vmlinuz-3.5.0-20-generic...
P: Writing config for /boot/vmlinuz-3.5.0-19-generic...
P: Writing config for /boot/vmlinuz-3.5.0-18-generic...
P: Writing config for /boot/vmlinuz-3.5.0-16-generic...
P: Writing config for /boot/vmlinuz-3.2.0-32-generic-pae...
Killed

5 minutes later, seeing that nothing was going to happen, I send a Ctrl+C, and here's what the process returned :

^CFailed to process /etc/kernel/postrm.d at /var/lib/dpkg/info/linux-image-3.5.0-20-generic.postrm line 328.
dpkg : avertissement : le sous-processus ancien script post-removal a retourné une erreur de sortie d'état 255
dpkg: trying script from the new package instead ...
Examining /etc/kernel/postrm.d .
run-parts: executing /etc/kernel/postrm.d/initramfs-tools 3.5.0-20-generic /boot/vmlinuz-3.5.0-20-generic
run-parts: executing /etc/kernel/postrm.d/zz-extlinux 3.5.0-20-generic /boot/vmlinuz-3.5.0-20-generic
P: Checking for EXTLINUX directory... found.
P: Writing config for /boot/vmlinuz-3.5.0-20-generic...
P: Writing config for /boot/vmlinuz-3.5.0-19-generic...
P: Writing config for /boot/vmlinuz-3.5.0-18-generic...
P: Writing config for /boot/vmlinuz-3.5.0-16-generic...
P: Writing config for /boot/vmlinuz-3.2.0-32-generic-pae...

Then it was stuck again. sending Ctrl-C returned the following :

^CFailed to process /etc/kernel/postrm.d at /var/lib/dpkg/tmp.ci/postrm line 328.
dpkg: erreur de traitement de linux-image-3.5.0-20-generic_3.5.0-20.31+lp1087622v1_i386.deb (--install) :
 le sous-processus nouveau script post-removal a retourné une erreur de sortie d'état 255
/usr/bin/os-prober: 112: /usr/bin/os-prober: cannot create /tmp/os-prober.RfwQJG/mounted-map: Directory nonexistent
/usr/bin/os-prober: 113: /usr/bin/os-prober: cannot create /tmp/os-prober.RfwQJG/swaps-map: Directory nonexistent
Examining /etc/kernel/postrm.d .
run-parts: executing /etc/kernel/postrm.d/initramfs-tools 3.5.0-20-generic /boot/vmlinuz-3.5.0-20-generic
run-parts: executing /etc/kernel/postrm.d/zz-extlinux 3.5.0-20-generic /boot/vmlinuz-3.5.0-20-generic
P: Checking for EXTLINUX directory... found.
P: Writing config for /boot/vmlinuz-3.5.0-20-generic...
P: Writing config for /boot/vmlinuz-3.5.0-19-generic...
P: Writing config for /boot/vmlinuz-3.5.0-18-generic...
P: Writing config for /boot/vmlinuz-3.5.0-16-generic...
P: Writing config for /boot/vmlinuz-3.2.0-32-generic-pae...

Then I had to send again some Ctrl+C with the same kind of returned messages. At the end, dpkg returned  the following :

Paramétrage de linux-headers-3.5.0-20 (3.5.0-20.31+lp1087622v1) ...
Paramétrage de linux-headers-3.5.0-20-generic (3.5.0-20.31+lp1087622v1) ...
Des erreurs ont été rencontrées pendant l'exécution :
 linux-image-3.5.0-20-generic_3.5.0-20.31+lp1087622v1_i386.deb
 linux-image-extra-3.5.0-20-generic_3.5.0-20.31+lp1087622v1_i386.deb

I hope this will help.