Unfortunately, at least in the United States, acquiring a copy of the Windows media is indeed a violation of copyright, as the person who made the copy was only granted copyright to make a backup copy for him or herself. Distributing the copy violates the copyright, as does acquiring a copy of the media from anyone other than an authorized distributor, OEM, or Microsoft directly. In this case, at least in the US, it *is* a correct reading of copyright law. Acquiring a copy from just anywhere is indeed a violation of the copyright on the contents of the media. Anyone making a copy for public consumption, because software in the US and Canada is considered a literary work, would need the requisite agreements with the vendor (in this case Microsoft) to duplicate and redistribute the contents and/or the media containing the copyrighted work. Considering all but the very large OEMs do not have the right to duplicate media for redistribution, and retail media explicitly does not grant you the right to make copies of the disc other than for fair-use personal backup, doing this is indeed a copyright infringement and would be actionable in court. The gray area, as with books, is whether or not you can give or re-sell actual media (including the documentation and COA) to another - in the US, and I believe in Canada, the rights of the OEM are preserved and you cannot give away or resell an OEM license and transfer ownership of the software without also transferring ownership of the original machine where it was installed. In other countries, however, OEMs do not have that pull and you can resell or give away OEM software (again, has to include the manual(s) and the COA sticker/product key as well) without violation of the EULA or any copyright. Retail of course can be given away or resold (transfer of the retail package, including COA sticker, product key, and media) without issue. However, downloading a copy from the 'net, especially if you don't actually have in your possession the original, is most definitely not legal in the US (it's debatable in Canada, although it would probably be considered legal on any serious challenge in a court of law). Whether or not Microsoft actually cares if someone is doing it is up for debate, as it seems they only care to stop large-scale piracy, but it doesn't make it less legal because they likely won't spend the time or money to stop you.
As to the OP's question, the last digits of the product ID are created when the software is installed, and are generated based on the product code and the channel ID, along with the actual product key input during install, the machine hash, and depending on the Windows version there could be other things affecting the generation of the product key as well. There's no way to easily track these back to product keys or channel installs, and as such you should go on the first portions of the product ID assuming you no longer have the COA and product key available to denote OEM, VL, Retail, etc. And as to acquiring all of the CD images to distribute, without a formal agreement with Microsoft to do this you would find yourself on the other end of a lawsuit shortly thereafter I would suspect if you are in the US. You could find legal ways to acquire the media, however, and if it is important you should consider your options (talk to Microsoft, they have programs to handle these situations).