What's the best WMV HD codec? And H.264?

[Delerue]

I'm trying to play some HD videos, but I think I'm doing something wrong. I can't play "Step Into Liquid" 1080p (WMV) well; there's a lot of stutter and no sound. Even though I play T2 (WMV too) perfectly. I'm using MPC, but I've tried WMP and VLC; no sucess. VLC freezes image sometimes, but at least runs the sound.

H.264 is even worst. I simply can't play any video well, even in 'low' (480p) resolution. I've tried renaming it to ".mp4" and play in MPC, but it still desgusting; stutter all the time.

I think there's a software problem, anyway. I use K-Lite Mega Codec Pack 134. And I have an Athlon XP @2.1 Ghz, 1 GB RAM (415 Mhz) and Ati 9800XT. Fast writes is enabled.

Any idea to make HD videos run faster?