mbsa microsoft baseline security analyzer issue with administrative vulnerabilities

I run XP with 4 uses.  I ran microsoft baseline security analyzer 2.1. It tells me there is a problem with IE zones. When I go to the result details it tells me the problem is with user S-1-5-21 ----. I fixed the other issues with the 4 people on this home computer but I do not know who or what is user S-1-5-21-----.  How do I fix that?  Is that something to be concerned with?

THX for the suggestions.