depends which country you are in
if you are in the uk you should take it back to who ever sold it to you
in the case of pcs if its a software problem and not a hardware one you may get charged
and ideally you should have went to the seller straight away not 2 different computer shops--as the seller
may argue that those shops have caused a problem
as far as i am aware the seller has the right to try to repair it 1st before offering a replacement
a lot depends on which company sold it what attitude they will take
if in the uk you should read up on the sale of goods act