You need to let your battery run down completely before charging, the best way to do this is to wait until the battery is fairly low, then restart the Laptop, but don't go into XP, do you have a 'Boot Disk' you can use or go into set up?Reason for this is that XP will go into hybernation to try and save power, so there will still be power in your battery,really need to run it down completetly, then Fully charge.
The reason that your battery isn't lasting so long is because of the way you are charging it, also, if you are now doing 'More Power' things with your laptop (i.e applications), that will run down battery faster.
Steve