Fails to boot with battery

I'm going to help a friend with her computer tomorrow, so I thought I'd post what I know of the problem here to get some suggestions before I take a look at it.

From what I understand her laptop was totally failing to start up, as in she'd press the on button and nothing happened. At the suggestion of another friend, she removed the battery and tried to start it. This allowed her to access safe mode, but that was all. Putting the battery back on results in the original problem.

As of now, I know nothing about the specific hardware. The computer is running XP and appears to be at least a year old, probably a little older. I couldn't even tell you the manufacturer.

I'm not really a laptop person, so I don't know a lot about their aberrant behavior patterns. The first obvious thought is that the battery is kaput. I'll also look around inside and make sure the connections are all in place. Apart from that I don't really know where to begin.

Also, to satisfy my own curiosity, what is powering the computer without the battery? Is there some small reserve motherboard power being tapped?

Sorry this is so vague. Like I said, I'm just trying to get a few general ideas for addressing this type of problem.

Ha, so this isn't just weird to me?

I would guess it is at least 2 years old, maybe as many as 4.