UK law...
Under the Sale of Goods act 1979 (amended),
"Goods Must: Fit the description given; Be of satisfactory quality; Be fit for their purpose; If not the retailer is legally obliged to remedy the problem."
Your contract is with the shop where you bought it, not with Apple.
Simply hand the goods back to them, saying that you reject the goods, and wish to have a refund. Ask to speak to the manager, and use the words "Sale of Goods Act 1979 Amended" and "Breach of Consumer rights" This usually does the trick.
Also quote the Sale and Supply of Goods to Consumers Regulations 2002, which states that you have the legal right to...
"choose repair, replacement, partial or full refund, or compensation if a fault appears within 6 years, if it is reasonable for goods to last that long."
If the shop refuses, contact your local trading standards officer (take the number with you and dial it in the shop)tell them that the shop is refusing your request for a refund on faulty goods.
Many shops are ignorant of the law, many ignore it. Many will try to tell you its Apples problem (it isnt). At the end of the day they just dont want to give you your money back. Good retailers dont argue, they just give you a refund. They know the value of repeat custom. I have never been back to places where I have had to argue my point. They have lost thousands in lost custom from me.
Sometimes you will have to stand your ground, and speak to managers etc.. Of course this should be done in front of all the other customers (not good for business). Sometimes you will have to walk away and send a Solicitors letter.
It may be that you have to take them to court, but you do NOT have to accept faulty goods, none of us do.
As you bought the computer on a credit agreement, then the finance company is equally liable for the cost of the goods. You can sue them too if you have to. You could contact the credit firm and let them know the situation, and tell them that they are equally liable under consumer law for the condition and suitability of the goods
Have a look at
http://www.dti.gov.uk/ccp/topics1/saleandsupply.htm and perhaps the "which" website.
If you are a member of which legal services then contact them. You may have legal advice with your house insurance.
Dont give up. I once had a Fujitsu Laptop which crashed every time I plugged a USB device into it. It was a design flaw causing a bus undervolt when USB powered devices were plugged in, but the shop claimed it as a characteristic of that model. A year later I got a refund of the full amount as I had no significant use from it. I used the money towards my first G3 PowerBook.
I have never been unsuccessful in a claim for a refund. But then the goods have always been imperfect, and the law has always been on my side. Sometimes though you just have to be patient.
Powerbook 17", 1.67GHz,DL,1.5GB DDR2. Mac OS X (10.4.3)
Powerbook 17", 1.67GHz,DL,1.5GB DDR2. Mac OS X (10.4.3) 
