Vista Tutorial - SP1 Servive Pack stopped Appearing in Windows Update

I purchased my HP M8200N PC just prior before the SP1 Service Pack was
released by Microsoft for the Vista OS. I held back installing same, since
I read many troublesome reports from different sources. I kept up and
installed all other recommended updates from Microsoft, except for SP1. My
Windows Update setting from day one was set as " Inform me of new updates
but let me choose whether to download and install them ". Up until that time
Service Pack 1 always showed as available for download and installation.
Recently I changed my setting to " Install updates automatically " without
changing the time schedule from when my PC normally is not powered on.
Since then I have noticed in the Update History of my computer that Service
Pack 1 and some other updates had made an attempt to install but failed.
Now, when I search for new updates, it informs me that no updates are
available. Returning to my original setting has no effect. How do I return
to get Windows Update to recognize that certain updates did not install? If
anyone has any suggestions please let me know. Thank you.
Dieter