What causes a window to close automatically when you are using the Internet?

Ah lots of reason might of crashed or you gout a virus and your computer unstable but if you tell me OS I can give you a list

Windows XP
Happens on Firefox and IE too

Oh that better k you may have a timer on do oyu have anti-virus do you ever se a blue screen pop-up or your computer trying keep you out of harm way by making yo u leave before it happen that what happened to me