Have you tried the following method to install the AC'97 sound drivers?:

Right-click on My Computer and select Properties. Click on the Hardware tab and then click on the "device manager" button. This will bring up the device manager. Right click on the device, and click on "Update Driver." Don't let the computer automatically scan/install drivers or connect to Windows Update. Then click next and choose "install from a specific location." Select the directory that has the drivers (if the drivers came in a zipped folder, you must extract it to somewhere on your HD).

Then follow the instructions. Hopefully that works. It always works for mine when the automatic installers don't work. Let us know how it goes.

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AMD Athlon 64 X2 4400+ (@2.6 GHz)
2x1GB OCZ Spec. Ops. PC3200
ASRock Dual939-SATA2
Antec TruePower 550 watt PSU
80 GB Seagate Barracuda SATA 7200 rpm
300 GB Seagate Barracuda SATA 7200 rpm
120 GB Maxtor ATA/133 7200 rpm
ATI Radeon x1950 Pro