I have not had a problem with the motherboard you have listed but if others have or havent isnt really the case. As it stands you have a non working computer and have every right to get the situtation sorted asap.
I think that before you take them to court one final letter firm and to the point may not go amiss. State that you have not had a working computer from day one and any dealings you have had from with the support service. Im not sure but to me it would seem you are within your rights to ask for a replacement product and not just a repair as it seems like the goods where not fit from day one. Also get in touch with trading standards and let the company in question know that if the situtation is not sorted asap you will be taking further action.