I had gotten a Windows 8 upgrade key with the purchase of a computer last summer. I hadn’t used it, so I wanted to upgrade a desktop my family uses. I ran the utility WindowsSetupBox.exe that you can download from the Upgrade Windows with only a product key page.
However, it failed quickly after choosing either to run the upgrade or to download it to media. I didn’t write down the error message, but it wasn’t very descriptive – it just said that something went wrong. It failed so quickly, it seemed like it wasn’t really trying.
So I downloaded SysInternals’ Process Monitor in order to see which files and registry keys the upgrade utility was using. Scanning through the output, I saw C:\users\<acct>\AppData\Local\Microsoft\WebSetup. I deleted that folder. After that, the upgrade utility started working as expected.
I noticed the dates on the files were from January, and that’s when I remembered having started to upgrade the computer from Win 7 to 8 and later canceling. Apparently having the old data there was causing the upgrade utility to fail.
I hope this helps anyone who runs into a similar problem.