Your system has infected whatever you download or click unknown link/CD/USB memory stick that contains hidden virus you never knew it existed.
Do your laptop fully protected from virus, malware, spyware, etc. by installing anti-virus protection software?
Do you update your anti-virus periodically?
Did your installed anti-virus scanned your system weekly or monthly?
Check out the existing threads at below:
Why Does My PC (XP OS) Shut Itself Down?http://www.nairaland.com/nigeria/topic-14792.0.htmlWindows XP Blue Screen (unmountable-boot-volume): Please Help!http://www.nairaland.com/nigeria/topic-97511.0.html