K-lite issue then mp4 issue

[Arch3type]

Hi, Lately I have been having issues with the playback of videos of pretty much all types.
When opened in MPC-HC (K-Lite) it would open, recognise movie length but not play the video.
I then opened the video in Windows Media Player and noticed the luminosity was present but the were abstract clusters of squares with green and pink hues over the top of the video (luminosity still visible).
So I updated to the latest version of K-Lite and the MPC-HC update, but it only provided a temporary fix.
At this point I thought I'd try a hail mary and re-installed DirectX(9.0c). No improvement.

So I uninstalled K-Lite and found XP Codec Pack and installed that instead (which contains a copy of MPC, ffdshow and others).
This fixed my problems with video playback, however, now my .mp4 files have no sound. It recognizes sound existing in the file but is silent on every program except Quicktime player which I loathe tremendously.

Please help.

WinXP SP3 DX9.0c
512MB Nvidia AGP
3.0GHz P4 Chipset
1.5GB RAM DDR1

EDIT: I can demux mp4 files and remux them into other formats and they play fine. But for something as common as .mp4 I should not have this problem.