laptop suddenly turn off
 

My two years old eMachines M6807 laptop had a problem recently. The problem is that sometimes during booting, after I select the menu to boot into Windows or linux, it boots but before the login screen appear, suddenly the laptop suddenly turn off.
It also suddenly turn off once yesterday when I am using Windows XP. I usually plug the computer to AC power.
What is the cause of this problem?
Is it the battery? harddisk?
How to find out for sure?

Last problem I saw like this was a battery problem. The terminals on my battery weren't making good contact and the system fans or my keystrokes would jiggle the thing just enough to shut it off. A new battery helped me, but I'm starting to think it might be the other end of the terminals because the problem is back. So I think I'm just SOL...