My problem is solved. I purchased another vendor's AV software two days ago and it loaded and updated without any problems. As I suspected it was NAV, which makes sense since the live updates caused the problems.
As an FYI, the reason SP3 did not install was because of a known problem with the automatic upload from Microsoft. A solution is to download and install from the desktop, or to update the registry file permissions. A friend of mine had the same problem when trying to upgrade to SP3 and the install from the desktop worked. I intend to upgrade to SP3 soon, but at this point, there is not the sense of urgency that I had. The solution is at http://support.microsoft.com/kb/949377
I appreciate all of your assistance in this problem and the advice on tools to keep my computer running better.
