When downloading Windows 7, you must choose between the 32-bit (x86) and 64-bit (x64) architectures. For almost all modern use cases, the 64-bit version is the correct choice.
Microsoft officially ended sales of Windows 7 on October 31, 2014, which means you can't purchase it directly from Microsoft's website anymore. However, if you have a valid product key, you can still download the ISO file from Microsoft's servers. Here’s how: windows 7 64-bit iso file download