Won't Let me Reinstall Windows

My computer is pretty slow and I'm looking to do a clean install and start from scratch. I'm currently running Win 8.1 I bought windows 8 a few months ago and my product key is not working anymore (says its invalid). Also when I click the reinstall windows option, it gives me an error message "some files are missing" and wont let me perform a clean install. Then when running the win setup to get the necessary files, it keeps telling me that windows cannot establish an internet connection.

Backup what you don't want to lose to external media. Boot from your 8.0 install media and do a custom install, not a reset or refresh. Then delete the partition Windows is installed to and create a new blank partition. Then install Windows 8.0 to the blank partition. It sounds like your trying to reset 8.1 with 8.0 install media, that won't work. Maybe this will help.
http://www.eightforums.com/tutorials/2299-clean-install-windows-8-a.html

I bought windows 8 a few months ago and my product key is not working anymore (says its invalid).

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Is this Windows 8 OEM or Retail?

What disk are using to reinstall Windows 8 or 8.1?