How to install Windows without a Windows CD

I have a computer Gateway computer that did not come with a Windows
installation CD. It has a D: Partition with a Restore to install the
computer back to the way it was when I purchased it. I came home today and
found the computer had crashed it said to run check disk. The drive did come
back up ok so far. As I write this on another computer I am running a backup
up the computer. I was told there is a way to make a Windows installation CD
from the computer so when I get a new hard drive I can format it and install
Windows. My question is HOW DO I DO THIS? Thanks for any help in advance.