You could have a virus. It could be a very nasty one that collects information about you and forwards it on to some party without your knowledge. Some of these viruses are not detectable by the standard virus protection software. To prevent viruses it's best to avoid Internet Explorer and Windows Live Messenger, as both are common sources of system attacks.Or more likely you accidentally hit the power button, or the system over heated. Any other software or hardware error could have caused the crash. If you saw a blue screen of death, then it was most likely a hardware or software error. Yup and also you can have an antivirus software enabled!