My friend is having a problem where for some reason ever since yesterday, when he boots up his machine, he is asked to select a user to login, but no users are listed to click on. He is therefore stuck at that screen. I suspect it is some sort of registry corruption. (He previously never got that screen and it only had a single administrator user that was automatically logged in when the machine booted). Is there a quick fix that can be done that does not require the complete reformatting/reinstall?