What is it running on? Vista, XP? It could be malware, I have recently seen AVG and other antivirus software delete the winlogon file as it is infected, this will prevent you from booting. Have you got a recovery CD that you can boot from? email me at paul_victoratbtinternetdotcom if you need further help. Note the email address needs changing as this site does not allow you to add it in
Paul
September 2010