I'm not 100% certain, but I don't think this is possible via the MC setup. Windows sets the permissions based on account type and I don't think installers can mess with that.
If you're happy with the above security settings, that's fine, I have just a little bit more to add though....
i mentioned above that library changes seemed to be taking. That's not the case. While they do appear to have taken, if you restart MC, any changes made in the previous session will be lost. Limited users can't change the library files. If you want that, you need to...
Log in as admin...
open any explorer window, click on "tools" on the toolbar, then "folder options". Click on the view tab, scroll down to the bottom of the options list in the lower pane and take the tick out of "use simple file sharing", OK your changes.
Navigate to you \program files\media center directory, right click on it, choose "sharing and security". click on the security tab and give the "users" account group full access. OK your changes.
Job done.
This will allow changes to the library, but will not allow files to be changed. I don't know how you use MC but you get the general idea.
I'm still very near to the bottom of the learning curve of windows security settings, and there may be a much simpler or more comprehensive way to do the above stuff, if anyone knows, do tell.
-marko