Copy-paste from a thread I made on OcUK...

Was restarting my PC randomly as I sometimes do, and when it tried to turn itself on, it turned itself off again. Now, whenever I turn it on, it lights up, makes some startup noises, then turns off after a few seconds, before trying all over again, endlessly. I initially thought the reset switch had gotten stuck, but I disconnected the reset switch and it still happened. I tried unplugging all peripherals, the graphics card, sound card, and hard drives, but nothing helped.

Did some quick searching and it seems like the PSU is at fault. It's a Corsair 520W PSU, but it's over three years old and has been put under quite a lot of load (overclocked i7, overclocked 560Ti). Is this definitely the problem because I don't want to order a new PSU without knowing for certain.

If I was to get a new PSU, does anybody have any recommendations? It would probably be around 700W, enough to give me some upgrade room in the future if I need it, but I also don't want to spend a lot of money.