I understand the Sale of Goods Act has a section covering the durability of a product which provides consumers with some compensation for products, which fail prematurely after the initial warranty period has expired.
Durability of goods depends on the amount of use, the condition of the goods (i.e. no apparent evidence of damage by the consumer), and the expected lifetime of a particular product.
There is sometimes a difference between the expected lifetime of a product (like the 5-6 years for a telly) and the length of time before a repair is reasonably expected. For instance, a TV may be expected to last 5-8 years (before natural replacement/supercedence or by the uneconomical cost of repair), but the set may be expected to need minor repairs after 3 or 4 years depending on use.
With computers, from my own experience I would think that PSUs would only be reasonably expected to last about 2 years, monitors about 3, CD drives about 2-3 on average use.
Remember that if you wish to dispute that a product has not been durable in accordance with the Sale of Goods Act, then after six months the burden of proof is on the customer. If you provide this proof, a retailer is obliged to carry out the repair.
isn't defined in the relevant legislation for obvious reasons - it would be impossible to do so.
Each case that comes before the courts is decided on its merits - the judge making the assesment as to whether or not the item failed inside or outside his/her judgment of what is 'reasonable' in the circumstances.