I installed a SIIG 4 port serial card to the WHS. Received the new hardware, notification, installed the driver. Received the 'new hardware notification' a few times even though the card was in and working. Rebooted, problem with 'new hardware' associated with the SIIG card gone.....Card does work. My Home Interaction system working just fine, audio and serial.
I had installed this same card on a previous install of WHS without problem. Card in same slot, (hadnt removed it), etc.