As you have a second system You should still have the EFI (BIOS) commands available in this case the easiest way is to connect your Windows Mac system to your MacOS Mac system via Target Disk Mode and then using a cable between them reformat the drive afresh with the proper GUID partition and do a fresh OS install from your good Mac. You could also launch internet recovery directly but if your internet connect is not very good I would pass on that.
Reference: Mac startup key combinations
So to do this just restart your Windows Mac pressing on the T key making it look like an external drive to your other MacOS Mac once you’ve connected the Thunderbolt cable between them. Then just run the Disk Utility to delete the partitions on your Windows Mac’s drive and then run the MacOS installer. If you don’t have it on your MacOS system just go back to the Apps Stop and click on the purchase tab to find it somewhere in the listing. (I do wish Apple would make it sortable someday). Download it and you’re good to go!
OK you don’t have a means to connect the two systems as your other system only has FireWire. Then you’ll need to create a USB OS installer drive following this guide: How to create a bootable macOS High Sierra installer drive if you have a different version of MacOS you’ll find a link to the others within this guide.
That gives you three different ways to recover your MacOS onto your system. But before you start any of these do make sure you’ve backed up anything important first!
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I was going to recommend you test going to the boot menu with another Hard Drive or SSD installed internally but this model has an expensive and proprietary SSD. Phooey! :-/
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