Your methodology is very sound, but it is highly unlikely that a Defender
update could keep your computer from starting. It is particularly unlikely if
the computer starts under battery power, but not when you use a power cord.
Try both a few times to make sure that is really the problem. If it is you
probably have a defective power cord. If you have another one, try that. If
not, talk to your OEM to see if you can get a new one.
Generally speaking, for these types of problems, the first thing I would do
is try to start in Safe Mode. That will disable almost all items that run on
start up. To do that, hit the F8 menu when the computer boots and select
"Safe Mode" from the Advanced Boot Options menu. If the computer comes up in
safe mode then it is one of the startup items that is causing the problem. If
it does not, the problem is likely to be hardware related, or it could be a
malware infection that has dug very deep into the computer. In either case,
you will probably need interactive troubleshooting.
If the computer starts in Safe Mode, try to use System Restore to roll it
back to a configuration that works. In the help window that pops up in safe
mode, click "What is System Restore." Then click "Click to open System
Restore" and use the wizard to roll it back to a date when you know the
computer was working. If that works, something you installed between then and
now broke it.
Your question may already be answered in Windows Vista Security:
> I installed the update for Defender KB915597 (Definition 1.29.8167.0) on the
> 28th, and this morning my computer isn't starting properly: it's hanging just
> before it starts to install my startup items, and I need to use the power
> button to turn it off. I'm using Vista on a notebook, and I have read that if
> my computer begins to behave oddly that I should look at the last thing I
> installed. But I see that one can't uninstall this Defender update. I did
> finally unplugged the DC power cord and started it on battery and it started
> fine. Does this make sense to anyone, and has anyone else had problems with
> this latest update?