download - Where can I legally obtain the 64bit version of Windows 8?

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Where can I download Windows 8 legally, from Microsoft?

No, I am not looking to pirate. I bought a key through the Upgrade assistant (for just $15 due to the upgrade offer), but it downloaded an iso file that was between 2.3 and 2.5 GB. Which doesn't make sense to me, because the Evaluation version of Windows 8 x64 is closer to 3.4 GB in size. I assumed the Upgrade Assistant would be intelligent enough to realize that it is being run on a Windows 7 x64 machine and by extension, download the x64 code.

UPDATE: I should clarify, because it appears this point was somehow unclear; I used the Upgrade Assistant to download the file and then with the same tool, converted to an ISO. I am concerned as to whether it's really the x64 image, because of how small it is in comparison to the Windows 7 x64 ISO. I'm currently on a very stable installation of Windows 7 and I only want to proceed to install Windows 8 if I'm sure I have everything ready. It's not that I am unsure how to install it, but I would rather not be forced to go through the setup only to discover at the end of the install that I have the x86 version.

Previously, I was able to legally download the ISOs (sans the keys, of course) from the Digital River host. I do not see an option to do that.

UPDATE #2: Because several users were being quite frankly obnoxious and rather unhelpful, I want to make sure this is fully explained. I was originally unsure if I had the 32bit or the 64bit ISO of Windows 8 and I was suspicious due to the size, and rather than barking at me to just try it, user "EKW" was kind enough to explain to me how to tell which ISO I have (which is still connected to my original question). Upon doing so, I have found that although I ran the upgrade assistant on Windows 7 x64, it still downloaded a 32 bit ISO, confirming my suspicions. I will attempt to try it again on a different machine.

If you open the ISO and check the \efi\boot directory, there should be an EFI file identifying the arch of the disc.

(bootx64.efi for x64, bootia32.efi for x86)

You can use your own product key to download an ISO:

1. Look at the email you get with the key. Download the linked tool.
2. Run Windows 8 Setup File
3. Enter your Product key
4. The tool now downloads the required files
5. After the download finished, select Install by creating media option and then click Next button.
6. In the following screen, you will see save ISO file option. Select ISO file and then click Save button to start creating the ISO file.

(Image source: intowindows.com)

Refer to Is there a place to download the Windows 8 System Builder ISO? There are no known legal Digital River links yet.

Which doesn't make sense to me, because the Evaluation version of Windows 8 x64 is closer to 3.4 GB in size. I assumed the Upgrade Assistant would be intelligent enough to realize that it is being run on a Windows 7 x64 machine and by extension, download the x64 code.

What makes you think you did not get the proper and complete version? Just the file size? Here's what the Improving the setup experience article has to say on the Building Windows 8 blog:

Constructing optimized download packages

The Windows 7 media layout for x86 consists of 874 files and 200 folders, with a number of redundant files both in the media and compressed within install.wim and boot.wim. To efficiently store and transfer the contents of installation media, we typically use ISO files. For example, an ISO created from the x86 client media is 2.32GB. In order to optimize for download in Windows 8, we take the required subset of files for the specific version of Windows being downloaded. After eliminating duplicates and compressing resources, the single-file size is 2.10GB (as compared to 2.32GB), a savings of 9.5%. After this optimized package is created we compress it using an improved compression algorithm specifically for Windows 8 setup, which provides an additional 28% savings. In this example (using the Windows 7 x86 ISO) the size of the download would be reduced from 2.32GB to 1.51GB.

I would advise that you actually try to use what you downloaded. If it fails, the download probably got corrupted somehow, instead of the Upgrade Assistant mistakenly downloading the wrong version.