I recently had some trouble with one of my older computers. I didn't use it much these days except for some basic web browsing or for playing really old games. The power supply died... one could say that he went out with a bang, quite literally. After some time, there was a loud noise, smoke and an horrible smell. I don't need to tell you that I didn't try turn it on after the incident.
Instead, I searched for an older 250W PSU I was keeping as spare parts, which I installed in the computer to test if the rest of my hardware was ok. That being said, I'm not sure what to do to ensure that everything is alright. The computers boots fine, and it works like it used to, although I did notice some unusual slowdowns.
Is there a way to test the components to be 100% sure that everything is fine inside after a short circuit/over voltage ? If I can confirm that nothing was damaged, I will probably buy a cheap 400W replacement power supply from Newegg.
Just the simple fact that it boots and runs is a very good sign. All you need to do now is check each component separately. See if it can read/write a CD (or floppy if it's that old).
Play a few games to see if the video card is ok. Verify the system sees all installed memory. Run a memory stress testing. Check that all the internal fans are spinning.
It was probably a capacitor in the power supply that blew. Electrolytics have limited life and they make a good sound when they pop.