Might come under Trade Descriptions Act too. What the hell are you doing using the card though?
you've been using it for two months! If you are not "computer savvy" then maybe the sensible thing would have been to get your computer savvy mate to check the components on the invoice before opening them all and building the PC.

There is a caselaw that states by accepting it even not knowing it was the wrong item, after a reasonable time it is deem acceptance. 2 Months is perfectly reasonable in anyone's eyes.