Windows Update is right - Belarc Advisor is wrong
This issue has been with us a very very long time. The moment you think you have it pegged, the shape of the peg holes change.
Belarc Advisor said my desktop needed 10 security updates and Windows Updates said none were needed. I finally found each of the 10 that applied to my XP SP3 system and installed them. I had to use Google to search for "KB*** update download" and then make sure I got the right one on the Microsoft site.
I bought a used HP NC6000 for a grandson and Windows Update said I needed SP3 which downloaded and installed fine. Then a check on Windows Update again said 18 updates were needed. They all downloaded but failed to install.
Belarc said I needed the same 10 that I needed for the desktop and I got them installed.
From now on, I will not use Windows Update.
The failure to install is apparently a common problem and I found lots of solutions that didn't work for me or anyone else.