it depends on how many user accounts you have on your xp machine.
if you just have your own account and no one elses, it'll boot straight to that account and right to the desktop.
but if you have multiple accounts, then it'll boot to win xp account selector thing (not sure what it's called). you just click on your name and it'll boot to that account (there's no need to enter a password).
hope that help.
I don't claim to know anything about everything, I just tell people what I know.
You choose what you want. You can have a single account go straight to the desktop or to the login screen. If you have multiple accounts I don't know if you can have one of those go straight to the desktop. If it did though you would just log out and back in as a different user. Passwords are optional.