My old desktop's PSU started misbehaving last week. It would post, run for a few minutes, then drop dead, and on a cold boot it sometimes wouldn't come up at all. The textbook signature of tired capacitors, and sure enough, with the cover off, two of the low-voltage electrolytics on the secondary side were domed at the top where they should be flat. One had the faintest crust of dried electrolyte around the vent.
Please mind the obvious: a PSU holds a charge after it's unplugged, the big primary caps in particular, and they will bite you hard. I left it unplugged overnight and checked across the main capacitor with a meter before I touched anything. Don't poke around in one of these casually.
The fix was unglamorous. Note the value and voltage and polarity of each bad cap, desolder, fit a replacement of equal or higher voltage rating and the same capacitance, mind the polarity band. I had a couple of suitable 105°C caps in the parts drawer, which is the whole reason I keep a parts drawer. Total spend: nothing, because I already had them. Total time: about an hour, most of which was getting the old solder to let go cleanly.
It's been running for three days now without a flicker. Whether it lasts another five years or another five weeks I can't promise, but it owed me nothing and the alternative was binning a perfectly good unit over forty pence of components. That's the bit that gets me every time. Not the saving, the waste avoided.