https://support.apple.com/en-us/HT201314
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Also if you have a copy of an OS X dmg file, one that shipped on your iMac or later, you can use a program on Windows called TransMac.
http://www.acutesystems.com/scrtm.htm
It has a 15day trial so that should give you enough time.
This will help you create a USB install disk from Windows. Just right click the USB drive from TransMac and choose Restore with Disk Image and point it towards your OSX file.
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You may 1st need to format your USB drive with GPT, the drive won't work if formatted with MBR.
In windows you can do this through the command prompt and diskpart.
Have your USB drive plugged in.
Open a cmd prompt.
diskpart
list disk
**** find the number of the disk for your USB drive
select disk 3 (<- where the number is the number listed for your USB drive)
clean
convert gpt
create partition primary
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I haven't tried this before but you could possibly download VirtualBox, install OSX virtually then you'll have access to the Mac App Store to download a copy of OSX, use Disk Utility or terminal to create a bootable USB drive then reinstall OSX on the computer.