George, I don't know the answer yet, but lets see if we can move forward.
You can get the latest definitions for your version of Vista from:
http://www.microsoft.com/security/portal
Check the right hand column. You'll need to know whether you are running
the 64 bit version of Vista or the 32-bit. If you are uncertain of this
answer, choose the 32-bit definitions. This will be correct even if you
know that you are using a machine with a 64-bit processor. (In this case,
the bittedness refers to the OS that is running, rather than the processor.
64 bit processors can run the 32-bit versions of Windows without any speed
penalty, and most of them are, in fact, doing so._
Let's see if that gets your definitions up to date. Once that is the case,
you could also re-try the update operation, just to see whether anything has
changed.
When you get the error message--how is the update being obtained? Through
Help, check for updates, in Windows Defender, or via Windows Update ?
The Event Viewer is probably most easily reached in Vista by hitting the
Start button, and typing "event viewer" without the quotes, into the search
box. On my Vista machine, that brings in one result up at the top, which
when clicked, starts the Event Viewer, after a UAC alert, and some
significant effort. Typing eventvwr.msc in that same search box also works.
All that said, I'm not sure what to look for. In XP, I'd look in the system
event log, going back to the time the error ocurred, and look for a log
record related to that error. If you can find sucn a log record, it would
be interesting to see what the content is--you can copy and paste back to
this forum.