Audio & video software: Choppy Video Playback - Old cpu

Choppy Video Playback - Old cpu

Ok, I'm actually trying to help my brother who's system is still functional but definately feeling it's age. For a while, he's been having an intermittent problem with certain internet videos, whereby they just won't play back smoothely on his system. Even if he downloads them, those videos will always have a severely choppy playback regardless of which program is used to play them. And in process explorer, every time one of these videos tries to play, the CPU utelisation will immediately jump on that program to completely use up all available CPU resources.
The CPU in question is a Athlon XP 1800+ 1.5 GHz with 2.5 GB RAM for memory, and an add on NVidia 5 series graphics card with 500MB itself.
My question is, besides shopping around for a CPU upgrade to last till the system can be replaced, is there any good software that can be used to reprocess video files to swap from processor intensive video codecs to ones that are easier on the CPU?