go to c:\windows\system32 and find a file called "hdaudbus.sys"
Right Click, 'My computer'
Left Click, Tab labeled 'Hardware'
Left Click, Button labeled 'Driver Manager'
Right Click, PCI Device with yellow exclamation mark
Left Click, Either 'Update Driver' or 'Install Driver'
once that file is installed go back to your disc that came with the mobo and install the asus driver. then you should be able to enable HD and install your card.
the m2n-e had this same problem.