1080p Movies Run Problems

Hi everyone!
I need help running @1080p movies with my old computer.
Is there any technical solution for this case?

Exactly the same movie (example .mp4 files H.264 encoded clips):
@720p no problems at all, runs perfectly.
@1080p can't play it smoothly.



SPECS:


1.PC

CPU: AMD Athlon 64 3000+ 2.01GHz
PCB: ASUS K8N-E
GPU: ASUS ATI Radeon A9600Pro 128Mb
RAM: 3Gb
OS: WIN 7 Ultimate x64 + SP1 (clean install last month)


2.TOOLS

CODECS: K-Lite Mega Codec Pack v10.32 full features configurated to MPC, WMP and BS.
CoreAVC Codecs v1.8.5 configurated with KMP.

PLAYERS: MPC-HC v1.7.3, WMP v12, VLC v2.1.2, BS v2.66, KMP v3.8


Please, don't need the 'buy a new pc' advice, I know I built this one at 2005.
I'm just asking for troubleshooting about this issue with this machine since I only pretend it for internet usage, watch some decent movies and eventually play a few old-school games.

My CPU usage when playing the movie
@720p jumps around 56% - 100%, but mostly uses +/-80%.
@1080p is always 100%

I don't know if it matters but the memory either way is always around 42% which is +/-1.27Gb
If it helps, here's some facts about problems noticed using different players @1080p:

MPC - Audio almost sync but turns off constantly through the film. Video full of grain as moving tiny squares (watchable but not smoothly).

WMP - Audio very low. Video full of big squares moving everytime (unwatchable).

BS - Audio very low. Video full of grain as moving little squares. Completly out of sync.

VLC - Video so choppy it's a slideshow and full of squares. Almost sync.

KMP - Video full of squares most of time. Totally out of sync.



Thanks for your attention.
I hope some brilliant mind can solve this problem.