Changed account name but can't change logon name

You cannot change a user account by simply renaming it. As you have found out, this makes a cosmetic change only. Create a new user account named the way you want. If you want to copy all your files/settings from the old user account to the new one, you need to do this from another administrative user or, since you have XP Media Center, the built-in Administrator account. If you don't care about the settings, you can just copy your data over. When things are to your liking in the new user account, you can delete the old misnamed account from Control Panel>User Accounts.

Copy Files to the New User Profile

   1. Log on as a user other than the user whose profile you are copying files to or from.
   2. In Windows Explorer, click Tools, click Folder Options, click the View tab, click Show hidden files and folders, click to clear the Hide protected operating system files check box, and then click OK.
   3. Locate the C:\Documents and Settings\Old_Username folder, where C is the drive on which Windows XP is installed, and Old_Username is the name of the profile you want to copy user data from.
   4. Press and hold down the CTRL key while you click each file and subfolder in this folder, except the following files:
          * Ntuser.dat
          * Ntuser.dat.log
          * Ntuser.ini
   5. On the Edit menu, click Copy.
   6. Locate the C:\Documents and Settings\New_Username folder, where C is the drive on which Windows XP is installed, and New_Username is the name of the user
profile that you created in the "Create a New User Profile" section.
   7. On the Edit menu, click Paste.
   8. Log off the computer, and then log on as the new user.

Note: You must import your e-mail messages and addresses to the new user profile before you delete the old profile. For additional information see 313055 OLEXP: Mail Folders, Address Book, and E-mail Messages Are Missing After You Upgrade to Microsoft Windows XP