Let me guess: BBC Planet Earth shows?
Good luck playing it. I can play it choppily on my laptop, a 2.13ghz Celeron M, but I can't come close to playing it to my satisification on my 2500 @ 2.3ghz Athlon XP chip.
Your best bet is to get a video card which allows some acceleration, like a 7800 or something of that nature. Many people are able to use the PowerDVD hardware acceleration to offload some of the decoding onto their video cards.
Other than a video card which can assist, nothing but brute force will allow you to play the files. I'm suprised that you can't play in relatively smoothly on your 3500 though. Are you sure that you have CoreAVC working with your media player? My 2500 will play it around 20 - 22 FPS, so it seems odd that you can't get at least 25 FPS or so.