That account is normally a hidden account. It disappears by default when
any new accounts are created. Assuming you have the password, if you have
XP Pro, hit ctrl-alt-delete at the welcome screen, that account should
appear and you can log on; if no password was created, just leave the
password blank and press enter. If you have Home Edition, boot the system,
start tapping F8, when the menu appears, choose safe mode and press enter.
At Safe Mode, select the account named administrator, enter your password,
if none leave blank and press enter.
Once in Safe Mode, you are their in the hidden admin account with access to
your files. Now, create a new user account for yourself with a name other
than administrator as the account named administrator is meant to be hidden
and is meant to be used only in the event the accounts you've created are
inaccessible; it's something of a backup to your the admin account you
create. It's essentially there for things you would not want to do as a
typical user, this is an added safeguard against outsiders or the
possibility some virus got loose on your system and try to execute a command
that might destroy your data.
Follow the instructions in the following Knowledge Base article for moving
your old files to the new user account you create and yes, this new account
can have administrator privileges:
http://support.microsoft.com/default.aspx?scid=kb;EN-US;q318011