Please read this conversation because it describe my problem too:
The only difference is that on my desktop (dc 5800 small factor) the ID for “unknown device” in Device Manager is:
ACPI\PNP0800
*PNP0800
“The drivers for this device are not installed. (Code 28) The installation of this device is forbidden by system policy. Contact your system administrator. To find a driver for this device, click Update Driver”.
When I try to update the driver with DI High-Definition (HD) Audio Driver for Microsoft Windows 7 (International) downloaded from HP.com (for dc 5800 small factor) the message is: “Windows could not find software driver for your device”.
Please help! Thank you!