Which windows cd computer repairer/shop use to reinstall fresh copy of windows by using customer's COA?

I was wondering which cd computer repairer use to install a fresh copy of windows by formatting the pc? I mean if the customer has a laptop with preinstalled windows with COA attached to laptop(usually this is an OEM version installed) and if their windows corrupted and they even dont have the windows cd which came with laptop and if he/she go to computer repairer shop which cd they will use to install. I went to 2 3 computer shop and ask them (by making story) that my laptop harddisk became crush and i want to reinstall the windows but i dont have windows cd. They said thats ok we will use our cd and use your serial key written on COA but i dont understand which cd they will use because i know every computer which use oem installed windows have their own unique cd key locked with they motherboard chip detection so which cd they will use? any generic OEM windows cd? if it is then is it legal to use generic oem windows cd and use customer's serial written on COA? specifically, if anyone know, is it legal in australia or not?