Where can I download Windows 7 (legally from Microsoft)?

The link should have been emailed to you with your receipt. More than likely it will not be an ISO but a set of ESD installation files. There is a tutorial here that explains how to convert to an ISO.

If the download link was not emailed to you I would contact customer service with your invoice number and ask to resend the information.

Solution for OEM Licenses

Microsoft's decision to take down the Digital River ISOs, leaving the Microsoft Software Recovery website as the only option, left OEM owners out in the cold, since in order to download an ISO today one needs the product-key of a retail version of Windows.

However, one can still head to the Microsoft DigitalRiver Mirror, a German mirror that has done an exceptional job of gathering up the Digital River ISOs and uploading them as torrent files. They have almost all the English versions, and a few in other languages.

This is entirely legal, as these ISOs cannot be used without a valid product-key for the Windows version that is downloaded and installed.

One would need a bit-torrent client, such as uTorrent, to download these files.

How to use the bit-torrent client to download such an ISO is well documented with screenshots in the article: Download Windows 7 ISOs, Legally and for Free.

THIS SOLUTION ONLY WORKS FOR RETAIL LICENSES, NOT OEM LICENSES

Microsoft recently launched a Microsoft Software Recovery for Windows 7. This allows you to verify your existing license key and download the current .ISO for the version of Windows 7 the license is for.

The website was created for the following purpose:

• Create a Windows 7 DVD for installation on a new hard drive
• Create a backup Windows 7 DVD
• Create a bootable USB drive with a copy of Windows 7

Step 1: Prepare to download

In order to recover your Windows 7 product or create a backup you will need to be able to download and save a large file (approximately 2 to 3.5 GB).

Make sure you have a reliable internet connection and sufficient data storage available on one of the following:

• 
  A computer

• 
  USB device

• 
  External Drive

If you are not ready to download, or are using a mobile device, an email link to download the product can be sent to you after you verify your product key. The link in the email is valid for 24 hours.

After you verify your product key below, you will download a disc image (ISO file) and use it to create a bootable USB or DVD. This will require ISO burning software such as the Windows 7 USB/DVD Download Tool.

Step 2: Enter product key + Step 3: Select product language

The next two steps will require you to visit the website. In the first step you enter your license key. In the next step you select the product language. If you need to know what your license key you can use the tool of your choice. I have found Speccy is reliable

Once you have down those last two steps you should get this:

Based on the comments from an article on another website users with OEM licenses will not be to be able to use this tool. Additionally it appears this service is only for those users who purchased the retail version of Windows 7. MSDN and TechNet subscribers won't be able to use this tool also.