If the old power supplies work, they can be used as... power supplies!
If you don't have mains wiring experience, I could not recommend any of the projects on the web that require you to take them apart and rewire them internally, but there is still plenty or scope for using them and keeping them sealed.
Some important points to note:
- A supply with its own power switch is probably best to start with
- You will almost certainly have to investigate which signal must be used to turn the power supply on (one wire is used for this purpose)
- You may have to put a load on the 5V rail to ensure the others are stable. If you can find a PSU that doesn't need this, all the better.
Apart from that, there are some parts inside most switch-mode supplies you can re-use, but frankly, not a huge amount.
There are:
- Several high current, low voltage rectifier diodes.
- A high voltage (generally 400V+) MOSFET
- An optocoupler
- Some low ESR capacitors (probably near end of life)
- A mains filter
- A SMPS controller chip, and a high frequency transformer that you probably will never re-use.
- Mains rated rectifier
- 400V (ish) capacitor (that will bite you if the power supply has been plugged in recently -- which can mean in the last week or so)
- Various assorted components.