How to make the debian system shut down with power off?

[James Ng Yuen Sum <james_nghk_debian_user@yahoo.com.hk>]

I have installed the debian system several times. In the first time,
after the installation, the computer can shut down automatically.(that
means the computer can shut down with the power off automatically when
you switch "shut down the computer" in the kdm.

However, in the second time, the computer cannot do the same jobs, so I
want to ask how to set the computer automatically shut down with power off

I have asked the same questions in www.debianhelp.org, and a viewer said
that it has to be done by configuring the modules, but he/she cannot
remember how to set it.