If I'm reading correctly your G5's optical drive has gone bad, and since you can rip a disk image from the install disc to a Windows machine with a working optical drive, you're wondering if there's any way to use that image to install on the Mac.
Assuming it's your optical drive that's bad, you're probably going to just have to replace it to install the OS. While software installers can be run from a disk image (so you could put that disk image on a thumb drive or external USB hard drive and plug that into your G5 and install from there), the full OS installer pretty much requires you boot from the optical disc it originally came on. You could theoretically create an exact duplicate on an external drive of sufficient size, but since you're starting with a Windows box with no firewire, this would be VERY challenging at best.
However, you can easily buy an optical drive that your G5 tower will use happily for under $30, so it would make a LOT more sense to just buy one and replace it. The installation takes five minutes at most.
Alternately, if that Windows box is a tower with a standard tray-load DVD drive, you could just pull it out and temporarily install it in the G5 to get the OS installed, then put it back. There's a good chance it will work fine without any drivers on the Mac.
If I'm misreading the problem and you're just trying to get a bootleg 10.5 off a Windows box to install on your G5, don't. That would be bad karma, if nothing else, and probably way more trouble than it's worth to save the cost of a legit copy of Leopard.
