Assuming it runs fine without a battery using the power adapter, my guess would be the battery's cells are too old and worn out to take a charge. You could always try the battery in a known good machine or a known good battery in that machine (to rule out some sort of problem with either the laptop or that specific battery)... but I'd put my money on the battery just being at the end of its life. It sounds like the battery is even too far gone for something like Battery Amnesia to try and recondition it to at least hold some sort of charge.

Considering how old the laptop is now... I highly doubt anyone is still making 'new' batteries for it. And even if someone had them, they're probably old stock that have been sitting in a warehouse for years and would be way past their prime. Some people claim that putting a completely dead battery in the freezer helps, but I've never tried that and can't recommend it.

Really, the only option left (assuming it is indeed the battery that's the problem) is to possibly re-cell the battery... either yourself or by paying someone else to do it for you. An internet search will turn up more than one step-by-step guide on how to do it, but it sounds VERY dangerous, so it's definitely one of those 'at your own risk' deals. I've looked into doing it myself, and it really sounds like something that's not for the novice or for the faint of heart, and is definitely best left to the skilled experts.

By the way, I wish I had those memory modules for my PB1400. Since the PB1400 uses special modules that only fit that model, I've yet to see anyone selling them.