How do I create a Mac bootable USB drive using Windows? My MBA 2012 with OS X 10.9.4 Mavericks won't boot anymore - it simply freezes after the initial jingle. I already tried resetting NVRAM and SMC, but to no avail. I don't have any time machine backups.

However, I still have a disc image of Mavericks sitting on an external hard drive, a USB stick and access to a notebook with Windows 7. 

I haven't yet found any tutorial on how to create a bootable USB drive on Windows in order to reinstall OS X on my beloved Macbook Air.

Any help would be greatly appreciated!
According to the first answer here, https://superuser.com/questions/383235/create-a-bootable-usb-drive-from-a-dmg-file-on-windows, there's a tool with a free trial called TransMac that can do it. Just make sure the USB drive is formatted with GPT and not MBR.

What might be easier, however, is that that model has support for Internet Recovery. If you boot holding Command-R and you have a WiFi connection, it can actually boot into recovery mode without a recovery partition on a drive (or even without a working drive). 

Having said that, your description of a crash right after the boot chime could signify a more serious hardware problem and you may not be able to boot anything. If you boot holding the option key down, the startup disk selection screen should appear. If it crashes anyways, you may be looking at a hardware problem.