Instead of redesigning or modifying the PC power supply, use the PC power supply as the 12V input. Then, make a 12V to 48V DC-DC converter. You can use a small ferrite transformer for this with push-pull converter, or you can use a non-isolated boost converter. Do a little search on boost converters if you don't know about them. Also, read about push-pull converters if you don't know about them. Then, start on the design phase. It won't be too difficult and you can always find us here if you need help.
Hope this helps.
Tahmid.