thats pretty much what i thought.
why do you need a new pc?
if its a good reason why dont you try asking your parents to get you an upgrade?
in any case, have you thought about what would happen if they didnt get you one? either you are now without a computer or they waste money getting it fixed and you're stuck with the same old pc except now they watch you like a hawk and dont let you install anything on the pc. or they could buy you something really low end. things dont always turn out well.
dont lie to your parents. just tell them the truth of why you want a new one and accept their decision.