pc logs on and then off

hi and thanks , i recently updated avast antivirus,and everything seemed to be fine .then my system started going a bit nuts !.so i uninstalled it,and installed it again.avast did its first scan auto ,then said needed to restart system so said yes .when my pc come back on it asked for password i entered it ,it said loading personal,went on to screen then said saving personal and loging off. i sant get it to work even in safe mode just does the same thing!.plz someone help!:confused