About a month ago our main PC started showing some worrying symptoms. It would wait about two minutes following power-on before its lights came on and it began start-up. Once started, the PC would then run normally, and could even be even shut-down and restarted again, without any problems, so long as the power was not switched off at the wall. It was as if the PC needed to soak up a certain amount of power before it could begin start up.
I recognized this as probable partial failure of the PC's power-supply unit (PSU) but, as it was only a minor nuisance rather than a catastrophe, I put off doing anything about it for several weeks. However, recently the delay between power-on and start-up had grown to over ten minutes, so I thought it was time to do something about it.
I opened up the PC and read the label on the PSU. It said: "ATX 12V. With PFC. Ball Bearing Fan. CHIEFTEC Model HPC-360-202. AC Input: 115V 8A, 230V 5A. Frequency 50-60Hz. Max Load 360W." I also used a ruler to measure the dimensions of the PSU: height 8.6cm, width 15cm, depth 14cm.
A quick search on the PCWorld web-site revealed that they could supply a suitable replacement: a "JEANTECH ATX 400W Power Supply Unit", of the same dimensions, for £50. After a quick walk over to their local store (only 2km away), I was back with the replacement in my hands in just over an hour.
I carefully read the WikiHow page on How to Diagnose and Replace a Failed PC Power Supply. I didn't have a power supply tester nor an anti-static wrist-band, as recommended in that article, but I was fairly sure it was the power supply, and, after the walk I was too sweaty to generate much static.
Removing the old PSU and fitting the new one was a bit fiddly, especially unplugging the various cables without breaking anything. I had to use narrow pliers to remove one particularly awkward plug. I also made a note of what I had unplugged from where so I knew where the new plugs had to go. I also took care to tie up any loose cable and plugs, so they don't snag any of the fans. After everything was done, and before I put the PC cover back on, I powered-up and switched the PC on on. It started first time. I checked all the drives and fans were working and then put the cover on.