it has come up because autopatcher has pulled in some patches, most probably dotnet, and this then make a new account, so now you have more than one account and xp wants to know which one you want to login with.
further updates made this behaviour go away, but ja, go into computer management, user accounts, and find the one called something like ASPNET and disable it
OR
start/run/ then type in
control userpasswords2
and untick "users must enter a username and password to login" or whatever wording they decided to use