Trouble with sound on win xp! Please Help!

This is my new rig: My old rig:
Mobo: Ga-K8N-pro-sli AbitKV8Pro
Cpu: Amd64 3800+ Amd64 3000+
Ram: 2*512mb ram KingMax 2*512mb ram Kingmax
Hdd: Seagate 80gb sata1 Seagate 80gb sata1
Cdrw: Asus Cdrw: Asus
Dvdrw: Lg Dvdrw:Lg
Fdd: Sony Fdd: Sony
Vga: Sapphirex1900gt Gigabytex800provivo
16pipelineactive
Psu: Chieftec 560w Psu: Chieftec 360w

I was forced to make a hasty and low budget upgrade. The old Vga had problems with the cooler and I had warranty on the system an so the shop where I purchased my system told me I could make an upgrade because they did not had a replacement for my Vga and breafly the only option I had was to change the MOBO, CPU, VGA and PSU. Due to limited funds this new MOBO was the only one I could afford. It has ALC850 Realtek AC97, 8 channels audio. My old mobo had AC97 6 channels audio.

I installed WINXP profesional+sp2, update all my drivers and I have problems with sound. FarCry, HL2, HL2EP1, FEAR, did not work. In FEAR when I disabled the sound in the games configuration utility it entered the game.HL2, HL2Ep1 after I installed RadGameTools started to work. With FarCray and Fear in mind I formatted my HDD and started all over. Now HL2, HL2EP1 and FarCry are working but FEAR does not.

I have to admit that the games and my WINXP are not originals but
before, on my old rig everything worked fine!

Wath could it be?
Could it be the audio onboard chip?
Some problems with the codecs?
WINXP itself?
Or the DirectX9.c
Wath version should i use? The one on the cd that camed with the VGA or the one that can be dowloaded from Microsoft? Can anyone knows a place where you can download the latest version of DX9c, the one that you are asked for validation?

THANKS