It should be displaying a username. Is it your sister's? Presuming it's not the result of some malware, you might try to use the System Restore function. You can set it back to a time when no password was required.
My sisters laptop, Win XP Pro/SP2 has worked for three months with no password set, just click on her name icon.
Today it wants a password entered before she can get in.
What has happened?
More importantly, what to do?
Any help much appreciated!