Cause - XP Service Pack 3 Gives Microsoft's HD Audio Driver Priority
Windows XP Service Pack 3 has decided that its HD Audio Drivers are superior, and does not allow others to be installed. If the Realtek drivers are installed and there is no sound, try uninstalling and reinstalling them. If they will not uninstall, follow this procedure:
1. Reboot the machine into Safe Mode.
2. Once loaded, bring up the Windows Device Manager.
3. Under Sound, video and game controllers, find and disable the Realtek HD Audio Device.
4. Then under System Devices, find and disable the device labeled 'Microsoft UAA Bus Drivers for HD Audio.'
5. Reboot the PC, again into Safe Mode.
6. Bring up the Device Manager again, navigate to System Devices to find Microsoft UAA Bus Drivers for HD Audio, then uninstall it. This will uninstall the Realtek HD Audio as well.
7. Under Sound, video and game controllers, uninstall the device 'Legacy Audio Drivers' as well.
8. Restart the machine in standard mode, making sure to cancel the Add/Remove Hardware Wizard.
9. Run the driver file from our website or the driver CD, and restart upon completion.
This should resolve the issue.