Well, with bank/debit cards in the US, and with many credit cards IIRC, it also applies to fraud. The proceedure would be to return your PC to the store (by leaving it there), and then you would contact your bank, disclose to them the situation, and then they would take action, by first crediting the money back to your account, and then suing the merchant.Who pays credit? I didn't think anyone would be able to reverse an arbitrary payment, however. Isn't that just for stolen cards etc?
A merchant giving you a defective computer and refusing to repair it, exchange it, or give you a refund for it in a timely manner would be an example of fraud, and so you'd be able to get a refund without difficulty.
A friend of mine earlier this year had a really annoying problem with Time Warner Cable. She paid a bill with them via debit card, and the money was taken, but Time Warner refused to acknowledge the validity of the payment and attempted to force her to pay the bill twice. She contacted her bank, who reversed the debit card transaction for her, and then simply cut Time Warner a check.
Of course, in Australia, things might well be more consumer-hostile. I've seen a number of disturbing reports of really consumer-hostile legislation and attitudse from Australian companies, so if you were to run into this problem there it might be more difficult to deal with. I would hope not, however.