DON'T DO IT!!!!
There is a reason the stores push so hard to sell the extended warranty -- they are insanely profitable -- more profitable than the computer! If they are so profitable for the store, can they possibly be a good value for you? Of course not.
In many big-box stores, salespeople earn incentives and commissions not on the equipment they sell but on the extended warranties. That's why they want you to buy it so badly.
Don't believe me? Here's some interesting reading:
The Consumerist: Fewer Consumers Buying Extended Warranties
ConsumerAffairs.com: 5 Reasons Not To Buy An Extended Warranty
It is also worth noting that many credit card companies offer automatic warranty extension just for using their card. For example, the Buyer Assurance Plan from American Express will double the manufacturer's warranty, up to an additional year, on many purchases made on their card.
Visa and MasterCard offer similar plans on many of their cards, so check with your credit card issuer before making a purchase. And be sure to read the fine print to make sure you are covered.