During the Joomla 1.5 installation process, you specify a password but the username is automatically assigned as 'admin'. Most people don't bother changing this which weakens your security as hackers already know half of your login details.
You can change your username as follows:
1. Login to Administrator
2. Go to User Manager
3. Click on the Administrator user record to go into edit mode
4. Change the username
5. Click Save
Remember your new username!