Security Update Requires Logon

A recent security update to Windows XP (sorry, can't tell
yu which update) now presents a new screen during logon
which requires me to select the user
(currently "Administrator" is the only identy installed,
and, since I am the only user, the only one needed).

How can I automatically log dircetly from boot to desktop
as it use to do, without this new screen appearing asking
me to log in as "Administrator", the only identity on the
computer?

Thank you in advance for your reply