|If you have been using the automatically-created "Administrator" account for yourself, it is normal behaviour for it to become "hidden" when you create a new administrator account.|
You really should not have used the automatically-created Administrator account at all as it's intended only for emergencies or to use for solving a problem. You should have created a new user account for yourself as soon as you got the computer or as soon as Windows had finished installing.
To "unhide" the Administrator account you have been using go here:
In the left pane, click "Windows XP Fixes".
In the main pane, click Enable Administrator account on Welcome Screen