I ran into the following issue while upgrading an older PC that finally
failed me. I could  not find any solutions online but after 2 days of
research, I finally found the solution and  thought I would offer it
(and circumstances behind my issue) for those who might encounter  the
same issue.

My Upgrade:
 New Gigabyte S-Series Mobo (GA-EP35-DS3L)
 New Intel Core 2 Duo CPU 3GHz
 New 4GB G-Skill RAM
 New 650GB WD SATA HDD
 New GeForce 8800GT Graphics Card

The only things kept original where the two DVD-Rs, 160GB WD SATA HDD,
CoolerMaster ATX case  and 500watt Norwood Micro Power Supply Unit
(PSU).

During the initial install of the mobo, cpu, RAM and graphics card,
there were no issues. No  issues after installing Win XP Pro, SP3. I
needed to get data off two other HDDs. At some  point, and I'm not sure
exactly where, but when I would shut down, the PC would shut down  and
the screen would turn off. But within a second, the power light, mobo
light and fans  came on. But the PC did not reboot. The only way to
turn off the PC was with the power  switch on the PSU. I was thinking
this could be an issue with my ACPI power mgmt. But I did  not make any
changes so that perplexed me. I reset the CMOS to factory settings.
That did  not resolve the issue. I noticed 5 lights on my mobo were lit
upon shutdown. According to  Gigabyte manual, it says my CPU is
overheating. This makes no sense since the CPU monitor  safety is set
to shut down before overheating and the CPU monitor displays a temp of
30c.

I searched for days on "XP won't completely shutdown". I now realize
that I was chasing a  symptom and not the problem. Thinking the PC
might be overheating, I shut if off for hours  (power switch on the
PSU). When I turned the power back on, the power light, fans, etc. came
back on. That got me thinking: At this point, the OS is not even
running. How can this be  the cause of my problem. It must be with the
mobo. Man, I hope my new mobo did not die. Then  I read one last volley
of "my system won't shut down" and the very last post said he had a
similar issue and it was the PSU. I decided to pull another PSU out of
a working PC and give  it a try. Sure enough, it worked fine. So, long
story short, my PSU crapped out on me. What  a PITA. Hopefully, none of
you will have to experience this issue.

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