Gordon Clark :
Suppose you buy a used laptop with no HD. You repair the laptop. How can you install windows without having to purchase a new key? Assuming the laptop came with a valid windows key when first sold.
Does the windows license die with the hard drive?
For OEM licenses:
If Win 7, the license key should be on a sticker on the case or under the battery.
If that is gone or not readable, too bad.
Buy a new OS.
For Win 8 and later, if it does not come with a drive and the factory reset partition...OR the Factory Reset DVD, you have 2 options:
Contact the manufacturer and purchase the relevant factory reset DVD
or
Buy your own OS.
There is a difference between having the hardware in running condition, and having the hardware + a viable Windows OS.
If the previous owner does not transfer that OS info to you, you're mostly out of luck.