To start with Leopard or Snow Leopard (10.5.x/10.6.x) is no longer available from Apple. You may find a copy on Ebay or a store some where.
Your system HD has a hidden partition (unless you took it off or installed a new non Apple HD) to repair your OS. If you upgraded it should have also ungraded the hidden partition.
If you can't get the OS to load on your MBA you won't have anyway to access WiFi to gain access to the DVD from any server. Generally you really can't access the DVD from your Windows system unless you have Both AFP services running and have Apple file system services running on your Windows system. If you have a second Mac you could do it from it sharing your DVD drive.
A better way is to get a USB flash drive and prep it on a working Mac as a bootable volume and copy over the OS install files from the DVD. Then use this flash drive to boot from and install your OS. Hint: bring your OS DVD and flash drive to an Apple store and they can help you put it together.
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