I suppose 8 weeks is long enough for the store to insist on a repair.
It's also a short enough time imo for you to say that as the PC had now been inspected and found faulty that it could not have been of mechantable quality in the first place and so to push for a replacement or refund on the spot. Sale of goods act. When the part needed is not obtainable quickly that adds further weight to your case.
To late now though to do much about it.