So watch out for the mysterious 28002 Error Unknown.  It really took me a little while to figure this one out.  I was updgrading a client's TFS server and they are using TFS Workgroup Edition and I was getting this annoying error when installing the KB919156 prerequisite to SP1.

Well, if you have 5 users (which is the limit for the workgroup edition) you are going to receive this error... Dave Glover writes a good explanation of this:  TFS SP1 Article

You need to remove one of the users from the "Team Foundation Licenses Users" group and add them after the SP1 install is finished.

Ed B.