About Windows XP asking for a username and password

This is one question I asked about an hour ago. But no one has posted a reply yet, so I would like to ask the same question again:

The other day I performed a routine scan on my Windows PC, but after reboot a login window appears on start up asking for a username and password, something I’ve never used. How can I avoid this screen? By the way, even if I click on OK button without entering any password, the system starts up. Thanks a lot for your advice.