How do i reinstall an audio driver?

I reformatted - windows xp home - now "no audio device found on system". I tried downloading the driver - downloaded - rebooted - "this system does not support the driver you are trying to install". Windows xp was already installed when i bought the computer so i do not have the driver disk. It is an Intel 845Glad P4 mainboard; Ac 97 sigmatel - the driver i tried to install is a sigmatel high definition audio. Why won't my system support it? *the only alteration made to my computer before reformatting was a PCI card installed. I don't know what to try next!