A service called the terminal service should be enabled for the username to be displayed in the username column in the task manager in the processes tab.
To enable this service do the following:
Go to start > run and type services.msc and press enter.
In the services window, navigate to Terminal Services and then right click it and select properties.
Select the startup entry as automatic and click the apply button.
Then to start the process from now itself(that is before the next restart) right click it and select start.
From now onwards the username entry will be available in the processes tab in the task manager.