computer does not switch on ,burning smell

The other day I try to switch on a computer to do a format but it didn´t switch on instead I smelt a burning smell coming from the back of the computer.
I quickly removed the power lead from the power supply to avoid damaging the computer even more ,I opened up the box ,reconnected the power lead and pressed the power switch and waited to see what was causing it.
Suddenly smoke came out of the power supply ,at the moment it seemed to be the culprit ,to make sure it wasn´t another component that was doing this I disconnected the power leads from the power supply to the other devices such as hard drive ,CD roms and floppy drive ,only the lead to the motherboard was connected I then reconnected the power lead from the mains to the power supply and pressed the power switch on the front of the computer again ,the same result happend smoke and burning smell.
knowing that the power supply had to be the culprit I removed it and connected a used power supply I had lying around .
The computer switched on ,beeped and booted into windows, all was fine.
It was lucky that only the power supply had died since it could have been a more expensive problem such as the system board or processor.