Fri Feb 06, 2015 9:11 am
It's... possible.
As opposed to impossible.
The Raspberry Pi 2 is starting to get to be the sort of speed where it can match a desktop PC, although not a fully tricked-out gaming rig.
You would need to know about soldering and electronics, because although the ATX power supply can be used, it can't be easily used, and electricity can be dangerous stuff, both to you and the Raspberry Pi. You would need to get a drill out, because you'd need some way to mount the RaspPi in the case and none of the holes will match. You would need to pass the video, keyboard and mouse cables in through the back panel, because the RaspPi does not have a standard ATX motherboard case panel.
And you would have a massive steel box containing a board the size of a credit card, which seems like rather a lot of over-kill and a bit unwieldy.
And as ghans said, it will not run any version of Windows (not even Windows 10) or any of the software you have.
It is possible, you're not the first person to contemplate it, but to do the job cleanly you'd spend more than the price of a good motherboard on connectors, wiring and power switching, and you'd end up with a mediocre PC that was ten times the size and weight that it needed to be.