I am having a similar problem with an Inspiron 1525 laptop using the current BIOS (A17), Vista updates are current.  The SHUTDOWN sporadically works.

Is this thread still active?

The Dell on line manual for the 1525 doesn't list the Setup (BIOS).  But, in many systems there is an option for "network" boot or boot from the Wireless or Ethernet connection if there is certain activity.  Disable any option such as this that you have and see if that takes care of the problem.  I've seen posts before (it's been a while) but this seems to be the general reason for rebooting on its own.

If you are using Hibernate or standby instead of the "power off"  the network boot also applies to that. 

Thanks for the information and possible solution.

On older PCs I have seen options such as "Wake on LAN". I am unable to find such an option in the BIOS settings (F12 during reboot).  I did however find an option dealing with the NIC (see http://support.dell.com/support/edocs/systems/sc1425/en/ug/f3593c30.htm ). I disabled the "Enabled with PXE"  and enabled NIC alone.

I have rebooted twice with no problem but since it is a sporadic problem ......   (I hate these sorts of problems. :-) ) I will update this thread in a week if the problem does not repeat.

I experienced similar issues with Vista.  No problemo on Windows 7.

I would highly recommend an upgrade to Windows 7 at your earliest opportunity (solves a LOT of problems, not just this one).

Thanks Rick but I do not want to put the money into Windows 7 . 

To continue my exeperience with this problem..  I still have the problem and on the last occurrence I found that removing the wireless mouse allowed the shutdown to occur.  Not sure if this was just a coincidence or not.

I have most recently been shutting down the wireless networking power prior to attempting to hibernate this appears to allow the hibernate to occur properly.

I have solved my wife's problem by taking her machine and getting her an HP laptop.  I hope my problem will be resolved by this power off of the wireless networking.