How do I create Win 7 re-installation disk?

As far as I know an install disk cannot be made from the installed version of an os. If your system is a prebuilt or a laptop the hdd should have a recovery partition which can be used to do a fresh install.
This is a bit of a slippery slope but if you do not have one of these and you own a legitimate retail serial or unused oem serial, you can download an .iso of an install disk and burn it to a DVD or copy it to a memory stick. A used oem serial would still be legal assuming its being installed with the same motherboard, though contact with m$ might be needed to get it activated.