Boot Log Off Looping

Hello,,
I Have a head scratcher going.
One of our daughters PCs has suddenly decided Its not going to work correctly.
Symptoms as follows.
Turn on computer
PC goes through the usual Bios song and Dance As normal
Gets to Welcome screen,, Welcome wave plays, then suddenly
The log off wave plays And It goes to the user log on screen with 2 options.
Admin,, or daughter.
Click either, Its Just loops back.
Tryd starting in safe mode. Still loops back
Tryd starting in last known good config.
Loops back.
Where do I Start?
trout