I was installing a new PC for a friend yesterday; after using the Easy Transfer Wizard (first time I used this and I can heartily recommend) to move their settings from their old XP system to their new Vista one I got the 0x800106ba Windows Defender error on start-up. Now there is a lot of frankly useless comments on this error on various forums, strange as the solution is simple. I suspect this is down to this issue being predominantly a problem that hits home user who are not a familiar with the internal workings of services etc. in Windows.
Anyway the solution is make sure the Defender service is set to auto-start. You get the error if it has not been started when Windows check to see if it is running, I assume as part of the security centre checks.
It is not that the forums are really wrong, they usually suggest a reinstall of Defender (which will reset the service start-up), but it is just that this is not easy to achieve on Vista where Defender is backed into the operating system not as separate install as it was in Vista.