How can I convince my parents about getting me a computer?

I am currently making money, so I plan to pay for the computer myself. I am looking on getting an EEE PC for $500 NZD. At Harvey Norman they are selling them, with no payments at all until 1st Feb 2009. That would be perfect for me, yet my parents just won't even consider it. They know I'm making cash, but they just get angry and ignore me if I bring it up. What can I do to explain to them? Why do you think they don't care? I have been wanting to get this laptop for god knows how long.