As you have seen, the Microsoft Windows 7 ISO download page does not work for OEM product keys (it does work for retail product keys).
Although the download page for Windows 7 Service Pack 1 download page (http://go.microsoft.com/fwlink/?LinkId=199583) has been removed, you can still download Windows 7 Service Pack 1 from the Microsoft Update Catalog: http://www.catalog.update.microsoft.com/Search.aspx?q=windows%207%20service%20pack%201
As far as a Windows 7 ISO file (with or without SP 1) is concerned, you may still be able to find one of these on Amazon. Just be sure to get one that corresponds exactly to your COA sticker (e.g., Windows Home Premium 32-bit).
Although I haven't tested it lately, the Microsoft Windows and Office ISO Download Tool from https://www.heidoc.net/joomla/technology-science/microsoft/67-microsoft-windows-and-office-iso-download-tool should still be able to get you an ISO file, with certain caveats. As I said, I haven't checked lately, so the following may be out of date:
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If you are willing to use an ISO file from August 2018 (5.5 GB), you can download that immediately. However, if you want the original version, downloads are quite limited. Notes below from the Heidoc support forum.
- "The download database on the server will be reset on the first day of every month. We will then generate one link per hour on a first come first served basis, whatever is requested, until all licenses are used up. The database is then again reset on the first day of the following month, and downloads can be provided again for some time."
- "The Aug 2018 Win7 downloads are "standalone" edition downloads (only come with Home Premium, Pro & Ultimate as separate editions and not bundled together) that include IE11 & .NET 4.7.2 but are available in US English language only. all post sp1 updates except the big KB3125574 rollup update are included."
- "I suggest you use the original version (NOT Aug 2018) since it is the official one and it can be unlocked to become a multi-SKU disc image. The only downside is you have to install all updates after the installation." https://www.wincert.net/forum/topic/14731-is-it-just-me-or-is-the-official-client-fixed-resolutioned/
- https://www.wincert.net/forum/topic/14688-question-about-win7-iso/
For additional details, see: https://www.wincert.net/forum/topic/14659-win-7-office-2010-monthly-download-days-starting-october-1-experimental/