No. The way it works is, when a customer brings his pc in and it requires reinstalation of the OS, they provide the genuine Microsft disk and key for their own pc's.
If they don't have a legit version to reinstall, don't touch it, or repair without installing an OS. Don't fall for lost it, dog ate, sister broke it. No original disk, no install, keep it simple and you won't have Microsoft on your doorstep.
You can use a spare hdd of your own to install an OS for testing, then remove it before returning it to the customer.
A lot will be OEM systems from Acer, HP Dell etc. with a restore partition and or restore disks.