Well, I have this same problem with my Windows 7 Pro. I am the only user (administrator), and I cannot add or modify any accounts through "User Accounts". The option is there, but when I click it, nothing happens.
However, I can add/delete/modify user accounts through the Command Prompt using the "net user
It's kind of a pain to have to do it in the command prompt, but at least I can do it. I don't know if this will work in your case, but it is the first thing I would try. I've spent countless hours trying to fix the problem so I can add/modify through the User Account interface, but it just won't work. Try the command prompt, and see what happens.