Laptop Computer suffering an auto-standby

Hi experts,
I need your concern and ideas regarding with my problem.
My Laptop computer suffering a serious problem. When I started to use my laptop and do something with the Internet it will suddenly turn to standby mode. Actually 2 to 3 hours of using the standby will attack and will automatically shuts down.
Every time I use the laptop the standby issue will always strike the computer.
I don't even know what to do with it because i don't have any knowledge to some troubleshooting issues.

Windows XP
Preparing to standby…
So please if some of you knows solutions to, my problem please post it. Thank You.
Chrish
