How are you trying to shut the machine down? A power button? A menu entry? halt(8)? Do all of these behave the same way?

All these behave the same way. Only cutting the power supply by pushing the power button for more than 4 seconds stops shuts down the machine.

This is a hardware problem or a configuration problen within the CMOS
... please note that the shutdown(8) command (executed even with the
menu entries with `shutdown -h now') signals the /sbhin/init program
to switch to the runlevel 6 and the last command from the users space
during this runlevel is `halt -p -d -f' at least if poweroff was not
disabled. This signals the kernel to send a poweroff signal to the CMOS
... and this seems not to work with this mainboard.

Beside this: it is a bad idea to call the `halt' comand instead of
`shutdown -h now' due to the fact that the halt command of other
UNIX systems do not know the option `-f' aka `force' like the Linux
version of halt which leads to a halt signal to the CPU.

It is not a hardware problem, since windows vista shuts down the computer without any problem. There is a problem in the shut down menu of opensuse 10.3

Belive me: it *is* a problem with your hardware regardless if windows vista
overwrites the CMOS settings and the linux kernel does not. You may try out
on the command line within e.g. konsole the commands

         shutdown -h now

or

         shutdown -P now

which is at least the same as done by the menu button (first is halt the
system, the second is power off the system).  You may also set within
/etc/sysconfig/shutdown with the help of the vi editor or with YaST with
the entries `System' > `/etc/sysconfig Editor' > `System' > `Boot' > `Halt'
the value for `HALT' from `auto' to `halt' to if halt works for your (this
does not poweroff but only halt the system).  Or if poweroff with the help
of the ACPI interface of your hardware works for you (which IMHO is broken)
you may force poweroff by choosing `poweroff' for `HALT'.

Beside this with the systems around here `shutdown -h now' works as it
should be even if called with the help of the menu.

*** This bug has been marked as a duplicate of bug 331747 ***