Windows 7 log off automatically

Im using Windows 7 ultimate 32 bit.. I have two users in my pc...one administrator user which im using and a standard user for my family..Before two days it started..I can access my administrator account normally but when logging in to the standard users it says loading and automatically logging off.. pls help me to get through off it..I didn't install any software...but updated my AVG antivirus only.. Thanks in advance..