Source

If you purchased a retail copy of Windows 7, chances are your installation disc didn’t come with SP1 included. That means if you want to reinstall Windows 7 at a later date you’ll have to reinstall from the disc, then download all of those updates again, a time-consuming and frustrating process.

There’s another problem too: should you ever want to perform a repair installation of Windows 7 by choosing the “Upgrade” version, typically to fix a major system problem, then you’ll have to uninstall SP1 first from your computer before doing so – more time wasted and hair lost.

The answer lies in getting hold of a copy of the Windows 7 installation disc with the Service Pack 1 updates already included. You can do this manually using a tool called RT Seven Lite, but yet again it’s time-consuming, fiddly and – you guessed it – potentially frustrating.

Alternatively, you can download a legitimate image of the Windows 7 disc with the SP1 files already included. All you have to do is download the ISO file here, then burn it to disc by double-clicking the file and popping in a blank DVD when prompted. Once done, boot from the disc to verify it works and label the disc accordingly. You’ll need the product key that came with your computer to activate Windows 7 after reinstalling from scratch.

Note that this won’t work if you have a version of Windows that came pre-installed with your computer. Also make sure you download the correct version of Windows 7 for your needs – to find out which edition (Home Premium, Professional or Ultimate) and type (32-bit or 64-bit) you need, click Start, right-click Computer and select Properties.
My PC came with Home Premium 64 Bit installed. I had owned Win 7 Pro 64 Bit so I performed Windows Anytime Upgrade To my old W7 Pro Key and it worked. I then after 2 days performed this Upgrade fix using the ISO that I downloaded from The Source. It worked without a problem and took about one hour and 4 restarts. At the end of the source story are links to all Windows 7 Flavors. This is a great tool! I lost no programs.