Vista - Windows defender error after SP2 install

I just upgraded my Vista 64 to SP2 and now I get the following message on boot up:
Windows Defender
"Application failed to initialize: 0x800106ba. A problem caused this service to stop, etc., etc.,"
restart does not help and I found the service, it is closed but do not know how to start it. I don't see an option to start. I tried to download a fresh copy of defender from Microsoft but I get told I don't need to install. I can't remove defender... it does not appear on my list to add or update software.
What must I do?

Run cmd.exe with administrative previlliges (right click on this file as admin) and type
sfc /scannow

if no results in cmd.exe runned with administrative previlliges type:

SC STOP Winmgmt
RD /S C:\Windows\System32\Wbem\Repository
SC START Winmgmt

If it also fail try this tool How to Fix Windows Defender

Hi click Start and type services.msc in the Start Search box and press Enter.
Double Click Windows Defender and select type as Disabled and Status as Stopped.
Then restart your computer and again open services.msc and select windows defender type and Automatic(Delayed Start) and status as Started.
See if it fixes the problem.

Flavius, that software on the link you sent me only good for vista 32. Thanks anyway for responding.

Kavita, it worked... I could not do automatic delayed but could do automatic and on restart the message was no more.

Thanks to both of you.

I m glad to hear that its working.