An interesting question. Here's what I think... As long as you do not destroy or otherwise change the partitioning of your hard drive, re-formatting the OS X partition and re-installing Leopard should not destroy or remove the Windows partition.
However, since Murphy's Law is more powerful than our human minds, make a backup of your Windows installation first using "WinClone". It's free.
Pop in your leopard disk and follow the install though, you can chose which partition to reinstall leopard on during the install it wont effect the windows partition in anyway