Building your own image allows you to "slipstream" (integrate) modern drivers and the latest security updates yourself. Best way to create Custom Windows 7 ISO : NTLite

: For keys not accepted by Microsoft's site, tools like the Heidoc Windows ISO Downloader pull files directly from Microsoft’s servers, ensuring the image hasn't been tampered with.

Before customizing, you need an official, untouched Windows 7 image.