I salvage a PC fonctional a few weeks ago but without the hard drive. I choosed to use it in my workshop only for watching photos and going to internet. I didn't want to spend any money on it so I decided to remove all the unusful parts like the dvd driver, card reader and some PCI extensions to keep only the motherboard, the graphic card and the Power supply. And for fun I made a new case smaller to fit on my shelf.
Step 1: Dissasamble the PC
It's very easy to dissassemble all the parts. I take some pictures to remember where were the connections. Then I unplug all the cables and unscrew just a few screws. It was done in less than 15mn. I put away all the other parts (I reuse the card reader for my PC) and keep only:
-the motherboard with the CPU fan
-the graphic card
-a 100mm fan
-the atx power supply
-the start switch.
Step 2: Cut the Base
Take measurements of the motherboard. Trace this measurements on a 10mm thick wood board, make sure to add 5mm each side to have more place. Cut the board and sand it. Place the motherboard centered and mark the supports holes and drill them. I drill more holes for some brackets, it makes the case stronger. Place the screws for the motherboard and add some nuts to raised the motherboard.
Step 3: Glue the Sides Panels
For the left, back and right panels I used 5mm wood board. Cut them in the right size, drill some holes for the brackets and glue them on the base. Use some clamps to hold them in place. Cut a big hole on the back panel for the 100mm fan and drill a big hole for the power cables. Place the fan with 4 screws. Then glue the back panel with the others.
Step 4: Make the Top Panel
The top panel is made with a sheet of steel from the case of the PC. I cut the sheet bigger to cut it more precisely after. Cut the holes for the motherboard connectors (USB, vga...). I used a dremel to cut them. Drill a lot of little holes for aeration. To make them straight, I use a paper with lines and a needle to trace them. Drill the holes with a 1mm bit then enlarge this holes with a 3 or 4 mm one. Bend the sheet and mark the place of the CPU fan and cut it. I place the panel in place and mark the end of the edges and I cut them with a saw. I add some holes for the power switch and the leds. Use some hot glue to put them in place.
Step 5: Connect Everything
Now connect the power supply, the fans and the switch to the motherboard. Glue the switch and the LEDs to the metal panel with hot glue. Before closing the case, test it by plug it in. If everything all right, close the steel panel. I attach the power supply on the top with some double face tape. If not, double check connections. I close the top panel with some glue.
Step 6: Make a Bootable USB Key
Without the hard drive you can still use it by booting from an USB key with Linux on it. I use this site to make one: http://www.linuxliveusb.com . They give you a lot of different Linux version like Ubuntu or Debian. Follow their instructions (I used some space for a persistent mode which allow me to save settings). Plug the key and you can now works with Linux!
That's it you have now a new PC for free.