One possible way is to get the hard drive out of the Mac, install Mac OS on another Mac computer and then put it back in Mac. If you got Mac, of course, which is not a case. But I assume that you have a friend who could let you use his computer for a hard drive formatting and installation. It will take just few minutes.
Second way, you mentioned a desktop PC. While it is a major hassle, you can install Mac OS on that PC - its called Hackintosh, and on the Hackintosh, you can use a lot of various software to install Mac OS on Mac's drive. If you tell me what kind of PC you use, I could give you some ideas. There are two sites which will greatly help you with installing Mac OS on PCs, as well as readily assembled installers. That's much longer process, usually may take half a day and also depends on the PC hardware.
Finally, there is a way to make Mac installer on PCs - it is not easy but possible. You have to use apps like Clover for Windows, though I myself never tried it.
You can also update your Mac Pro later:
mac-pro-cpu-compatibility-list.1954766
First I need to start by saying this is my first Mac. I bought it from an e-scrap recycler. It is a Mac Pro 3.1 with dual xeons. It did not come with an operating system and I have no other macs to try and get a recovery bootable USB from. I know that the latest official supported OS is El Capitan. I got a .dmg file and have tried various programs to burn it to a USB using my Desktop PC. So far I have had no luck. Programs I have used are.
UUbyte dmg editor
Transmac
PowerISO
If anyone else has some tips it would be greatly appreciated.
