I have had the same problem as others in this group with XP randomly
asking for a password. I have two user accounts one with a password
and one without. It is annoying and sometimes logging off and on fixes
this or rebooting will.

The last time it happened my wife was on the user account without a
password and I tried logging into the account with the password and I
couldn't (password wouldn't enter as before). I logged back into the
my wife's account. I then used Process Explorer to see what processes
were running that may be causing the problem and I found nothing
definitive. I then stopped all the processes running under the
password account thinking I could log back in that way.

Here's the scary part, when I switched user I didn't get the login
screen I got a blank desktop with the password accounts wallpaper (no
icons or task bar). I then hit CTRL+ALT+DEL and went file/run/explorer
and guess what, I was into the account???!!!???

If someone could find out how this happens you may be able to log into
any password account this way.