One of my computers had a PSU problem a while ago. It sat for a while, and after turning it on for the first time, the power supply did a hissing sound. It managed for a while (2 months or so), until it would turn the computer off every 5 or so minutes. Thank god for spares. Now I've got a 450w powering a single core Athlon 1.8GHz, and a GeForce 6200 :D