NO audio output device is installed

Hello,

I have this laptop computer bought from ebay. It is Fujitsu/Siemens with 2GB RAM, Intel core 2 Duo CPU T5250@1.5Ghz. Initially, windows 7 enterprise was installed and the laptop works stable except the audio system. The speaker symbol on the task bar shows a red cross sign and when I put the cursor on it, it says 'No audio output device is installed'. And there is a yellow mark on the driver in device manager. I tried almost everything which I got from online forems including;

1) updating the high definition audio driver results: Windows found driver software for your device but encountered a problem installing the driver software for your device-The device can not start (Code 10)

2)Disable and then enable the device: no positive result

3)Uninstall and installing the latest audio driver from the manufacturer website: device driver software was not successfully installed: High defination Audio device...Failed.
4) A clean install of windows ( this time I installed Windows Vista home premium, the manufacturer pointed out that the driver is not compatible for windows7). the same problem though

5) Updating the BIOS. I found out that the BIOS is an older version and I upgraded it using the window version of the flash-BIOS, the result: windows could not start so I reinstalled windows and this time windows could not complete installation. So I decided to downgrade the BIOS to the earlier version, then windows start functioning but I still have the sound issue.

I am now running out of hope so please HELP. Am not An expert on these but i can follow step by step instructions in a moderately technical terms.

Thanks in advance.