This article may be of some interest.
http://www.pavouk.org/hw/en_atxps.html
This is the circuit of a common ATX power supply.
It uses a TL494 switcher chip which was / still is a very common control chip for these types of power supplies.
As long as the power supply you wish to convert has this chip or a similar one , irs pretty easy to make the output adjustable.
The 5V and 12V rails are regulated, but as there is only main transformer the outputs of the 2 rails are not totally independant.