2 mo. old 11" Air - guest account suddenly acquired a password which I did not set.

A week ago (+/-) Itook my 2 month old 11" Air to work. Took it out of the case - I first checked the battery - 80%. As soon as I clicked the track pad the thing went dead and would not restart. I plugged it in and tried to boot it. Nothing. I know - static - but I was working at a computer work station with a computer, two monitors and a printer all of which I am constantly touching. Took it home at the end of the day, when I took it out of the case it was hot. plugged it in and tried to start it - nothing - I mean nothing, no bong, no lights , no log in screen. Looked dead. Unplugged it and went to bed - wasn't going to leave hinky electronics plugged in while I slept. Next morning I open the lid and there's the littlle 'battery needs charging' icon so I plug it in for a while and it seemed to work fine. On the weekend took it to the local Apple repair depot and had them run the diagnostics and it passed with flying colours. But! When I back up to Time Machine the tool bar icon doesn't spin (it does do a back up) and my Quest Account has aquired a Password that I didn't set and I have no clue as to what it is. I'm suposed to be the administrator on this machine , how do I either delete or reset it?