I have used Belarc quite often and find it quite useful.
I have only used SIW on about 2 occasions I did find it very good and found that it's reports seem a lot more user friendly.
The missing updates could possibly be updates you have hidden (Do not show again)
Or updates that have not installed properly.
There should be a update number with each of them, so you could try going direct to the MS site and download them again.
I now use Avast on my Vista PC and it'is recognised OK.
I think Maybe that's just one of those things that Belarc doe's not recognise.