You might need to log into a Windows computer, but not with your current username.
1. After the computer boots, you will press Control + Alt + Delete to log on.
2. Press the Switch User button.
3. Press the Other User button.
4. At the next screen enter the other username and password.
Note: if you are logging in for the first time with your new firstname.lastname format username, you will not notice any change in your files when you are log in - your old profile will automatically merge with your new profile.