I replaced the PSU in a friend's PC twice in about six months. Both times, I didn't have a spare so she was stuck with a PC World £49 job. I now keep a spare in the cupboard - I used it on mine a couple of days ago.
I think what happens is that the fan packs up due to dust and muck in the bearings, whereupon the PSU overheats and fails.
I have a theory (in the early stages of being tested) that a higher power rating supply won't get as hot if the fan dies, and may live longer. I have an extension to that theory that a big enough power supply won't even notice the fan not accling. That will be the next thing I try.
Another friend's PC died about three months ago, and his "usual" fixer couldn't diagnose it so he ordered a new PC online from the same firm as the original. He showed me, with pride, the spec of his new machine - truly done up like a "Christmas tree" with goodies ... and a 250 watt power supply. I pointed out that the stuff he'd loaded would pull at least 400 watts, and he was heading to an early BANG.
The subject turned to his old PC - from the same supplier. We investigated and found a (dead) 200 watt power supply. It now has a 450 watt supply, is working perfectly, and he cancelled the order for the new one. When the firm got shirty, he pointed out that the spec they'd "recommended" to him was almost guaranteed to fail since the PSU was totally inadequate and he'd therefore lost faith in their product and their guarantees. Their answer was that for an extra £50 he could have a 450 watt PSU. The order stayed cancelled!