its really just a matter of changing your power profile around and tweaking some other random settings, like cpu throttling.
my laptop has a dual gpu setup an hd 3200 for power efficiency, and a much more powerfull hd 3650. the factory setting was to automatically switch gpus when on bettery, which caused games to crash. the setting for that is in the catalyst control center. last i checked hp used nvidia so it may be a different config all together.
i dont know if xp has power profiles the way vista does. but my power icon in my tray, when i click on it produces 3 power profiles. my system came with several presets, but i deleted them all and substituted my own. i got one that has all the settings rigged for batterly life, low screen brightness, cpu throttles set at 0 to half and gpu in power save mode. then i have one thats sort of in the middle, it allows throttling of the cpu and i keep the brightness down, but the gpu is on full. good for playing things like tagap or old games on the go. then i have a full on high performance mod for when i want to play crysis at the bus stop. it gives me about an hour of play time before its toast. ive set them so that i have to manually change modes.
when you go setup one of these power profiles, you get a huge list of parameters to tweak. so dont expect to slap anything together in a hurry.