What you've been talking about are electrolitic capacitors, they are the ones that have chemicals inside and can leak. Electrolitics tend to like being used, the voltage at which they leak charge tends to drop the longer they don't get used. When you do use them sometimes they will leak charge excessevely and get warm. Large ones have vents and simply leak, little ones can explode speading electrolitic gunk everywhere. I recommend that you run the computers about once a year and let them run for about an hour to avoid this problem.
This does vary with the quality of the cap, good quality ones can last along time with no problems, but others don't. It also depends on how close the working voltage is to the rated voltage, the more a electrolitic cap is pushed the more likely of having problems.
I've seen several caps over the years explode when equipment firsts gets turned on after many years of not being used. It's fairly rare but it does happen. It's a good thing the large ones have vents, otherwise that could make for a big bang.