Can't shutdown computer

It seems ACPI or APM are not working properly. Are they enabled in your kernel? Are acpid or apmd packages installed and enabled?

You should also consider to change source package related to this ticket. I don't think yelp is involved :)

acpid and apmd packages are installed. Do I have to enabled them and how can I do it ?

Do I close this topic here and open a new ticket elsewhere ? But where ?

In order to modify source package, there's no need to close this ticket. It can be done by selecting "Change source package" and choose a new one. The best choice for this one is "ubuntu".

If you want to know if apcid or apmd are enabled, you should use
ps -A | grep acpid
ps -A | grep apmd

I cant help but wander if it is worth describing your motherboard, and if you know it, the chipset on it. It may be that there are known (or unknown) issues with it - in which case it would be time to file a bug report.

Sorry, but, could you explain what
ps -A | grep acpid
ps -A | grep apmd
means.

Must I write this on terminal board ? Or is it a press key, ... ?

I change Source package to "ubuntu" one.

Motherboard and chipset ?
Do you mean, to give you PC information results from a command like "cat /proc/cpuinfo" for example ?

Yes, the ps commands should be typed in to a terminal.

When i say motherboard and chipset, I mean which brand, and which models are they? For example, my board is a Gigabyte GA-k8ns-pro with an nForce3 250 chipset.

There's no noticiable difference of performance between the i386 kernel and the i686 one. It could be even slower. In the next version of Ubuntu, the i686 kernel will be obsolete.

Have you installed the `linux-686' package?

On terminal, the command returns :
"ps -A | grep acpid" => nothing
"ps -A | grep apmd" =>
4560 ? 00:00:00 kapmd
4571 ? 00:00:00 apmd

I have not succeeded to find the way to catch motherboard and chipset information. Could you describe ?

Yep, I've installed ubuntu from i386. And I confirm it works well with i386. Computer shutdowns normaly. After 3 months, I decided next week to install i686. With this version, it seems to be faster and better for streaming radio listening. But it can't shutdown alone.

I could decide to stay with i386 or try to find a secured solution to solve problem on i686.

In order to enable acpid you should launch the following command:
sudo update-rc.d acpid start 10 1 2 3 4 5 . stop 10 0 6

You may also want to disable apmd:
sudo update-rc.d -f apmd remove

I think that my response was confusing for you - I did not mean that the PS commands were in any way related to the motherboard and chipset information, only that those commands would need to be typed into a command line.

I suggest you try Luca's suggestions first - which also need to be typed in at a terminal.

If (and only if) those are not sucessful, can you post the output of "lspci" here, maybe we can use that to determine which hardware you actually have.