If I understand you corrctly, you want to change a username. The question is, do you want to change your system username, or do you want to change the "SYSTEM" username?
If you want to change "SYSTEM", then you cant. That user account is hard coded into Windows and is used for certain low-level operations that requires even higher access than Administrator.
If you want to change your own user name, do the following:
- Click the Start Orb
- Click "Control Panel"
- Click "User Accounts and Family Safety"
- Click "User Accounts"
- Click "Change your account name" (You may need to respond to a UAC prompt)
- Type the desired account name in the box provided
- Click "Change Name"