KB2641690 is an update to the Microsoft Untrusted Certificate Store but it not detected as required by MBSA.
It is not installed on my machine and does not appear to be superseded by anything.
I would install it if that's what your asking, its dealing with certificates that need to be moved to the Untrusted Store for your protection. I find it odd that MBSA says its not a required update. It doesn't matter that your using IE6, the Certificate Store is the same no matter what your IE version is.