If this is the built-in Administrator account, then the tutorial below can help show you how to rename it. Built-in Administrator Account - Change Name
If this is the default administrator account that is setup during the installation of Windows 7, then you will not be able to change the C:\Users\(user-name)
folder name. Instead, as you found out, you can only change the name for the user account for how it appears in the log on screen and Start Menu. The user accout will then have both names associated to the account. The only option for this it to create a new administrator account with the name you want, that's not already associated with any other user account, copy your files over, then delete the old account from within the new one.
Hope this helps,