Don't know whether this will help you or not, but I have a post open on
similar lines see post "Win Defender non start?"

For some reason, don't ask me why, but my defender is now starting 8 times
out of 10 (I'm logging it) after I re-checked some of the option boxes, see
my post. On the non start occasions, I am manually starting with no issues. I
still don't understand why though the security centre is not giving warnings.
Be interested to know if this helps you though!
--
W4 me


"Bquinn" wrote:

> No answers yet nor have I found a solution. I am not going to reinstall the
> Vista since everything else works and works well.
>
> Bquinn
>
> "mhenry" <mhenry@discussions.microsoft.com> wrote in message
> news:90784944-184F-4022-85B3-3E36137AD137@microsoft.com...
> >I have the same issues with my system. Did you ever get an answer to the
> > error issue?
> >
> > "Bquinn" wrote:
> >
> >> An interesting error occurred on my desktop with the Vista 64 bit during
> >> the
> >> first part of December '07.
> >>
> >> For the first time since Vista was loaded (about 1 October) I thought
> >> that I
> >> would start Windows Defender and scan the computer. This would be the
> >> first
> >> time in over two months. I got the following message:
> >> -----------------------------------------
> >> Windows Defender ---
> >> Application failed to initialize: 0x800106ba. A problem caused this
> >> program's service to stop. To start the service, restart your computer
> >> or
> >> search Help and Support for how to start a service manually.
> >> -----------------------------------------
> >> So I rebooted. Got the same message. So I went to the various places on
> >> the internet trying to find a solution to start Defender. None have
> >> worked
> >> so far. There were a lot of threads on XP, but I only found one on
> >> Vista.
> >>
> >> I even uninstalled MS Update <KB931099>, which was suppose to be a
> >> complete
> >> new Win Defender install and then reinstalled it again (with a reboot in
> >> between). No soap either with this change. Everything works OK on the
> >> desktop so I have gave up to find a solution at that time.
> >>
> >> I checked my laptop (Vista 32 bit) and defender works as it should. I
> >> could
> >> always go back to the repair of Vista on the desktop and I might do that
> >> sometime in the future. Any help or comments would be appreciated.
> >>
> >> Thanks!
> >>
> >> Bquinn
> >>
> >>
> >>
>
>