There is nothing whatsoever illegal about downloading CD images for a version of Windows for which you have a legal license, the "OEM" specific license is nothing more than a qualification for the maker of PC's to get a discount on the licenses, you do not pay for the bits on the disk you pay for the right to use them, if you have a legal license you have a legal right to use a disc replicated from an image regardless of it's source for that specific version of Windows, such as starter basic etc.. etc.. you can not install a different version of Windows other than that which is specifically allowed for your key.
The discs say on them "Do not make illegal copies of this disc" It does not say you can not make legal copies, and using someone else backup is still legal use because you have a license which permits you to use the software. This is why no Windows install disk has come with any kind of hardware protection since windows 95, it is trivial to find such images and I've had to for several systems where backup discs were lost.
From a copy of a disc which did not come with my PC (just the disc) my machine is Windows Genuine validated, that's as much proof as anyone should need. I did a clean install from an OEM disc to avoid the bloat ware that comes on the maker supplied restore discs.
The EULA of Windows software for my machine does state that publishing copys of the software for download is illegal, however downloading them and using them for backup purposes is not.