Components lose efficiency over time; their overall impedance can change from heating for instance, etc. Regular cleaning will help keep dust from building up and stressing out the components from heat. Monitor your power rails from time to time too; when your power supply's load regulation starts getting really high (i.e. your voltage levels drop more than normal when running a CPU-intensive task, for instance - these tend to result in system freezes and reboots) that's when you know your supply could be on its way out.
Cheaper caps tend to have higher equivalent series resistance (ESR) and lower max. operation temperature ratings, and using one of the fundamental power equations P = I²×R (also V²/R), a lot of ESR can result in lot of power (heat) dissipation across the cap and reduce its operation life. Usually these are one source of failure, but other failing components can cause magnetic devices (e.g. inductors) to saturate by requiring more current from the input to maintain output voltage levels with a given load. This saturation in turn causes greater stress on semiconductor devices such as power transistors and diodes, reducing their operation life as well. In short, supplies that use cheap or electrically-underrated components can result in disaster.
If you do end up wanting to open up the supply and replace components, just make sure the caps are discharged first (namely the large electrolytic ones at the supply's input and ouput). Also, just visually inspect any electrolytic caps to make sure they're not bulging.
As far as selecting power supplies go, check the current ratings on each voltage rail - the more the merrier, but then again, be cautious about ones with cheap prices.
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