a clean install would be the best.
i would recommend windows 8.1, you can download an iso from microsoft and you can use your win8 key to activate it
i never did that so i dont now much about it maybe this can help:
http://dellwindowsreinstallationguide.com/a-clean-install-of-windows/microsoft-product-activation/installation-of-windows-8-1-with-a-windows-8-0-retail-product-key-notes/
http://www.howtogeek.com/187525/how-to-perform-a-clean-install-of-windows-8.1-with-a-windows-8-key/
i think someone else can answer that more detailed.