I was always under the impression that once you installed Windows, put in the key, and then if the label or key information is lost, you'd never be able to get it back. And sometimes computers come with Windows installed, but the license key is no where to be found.
It is nice to be proven wrong. I came across an article called How to download the last three versions of Windows officially at no cost.
A utility called the Belarc Advisor - Free Personal PC Audit can be used, among other things, to recover the original license key used to register the system. This can then be used to re-install the system.
The advisor creates a local file which can be viewed in you web browser. Information is not transferred over the internet. Information sections provided by the software include: Operating System, System Model, Processor, Main Circuit Board, Drives, Memory Modules, Users, Display, Bus Adapters, MultiMedia, Virus Protection, Communications (network cards), Other Devices, Network Map, License Keys, and a Software Version List.
The manufacturer's recovery solution can be used, or for recent versions of windows, .iso files can be downloaded from Microsoft. There are some caveats, some of which are listed in the comments of the article to which I linked earlier.
These are the downloads: