You cannot rely on Windows.old folder as a backup method, but must backup your files externally before install.
Since you have to do this anyway, you might as well wipe the HD of all code to get the cleanest possible install:
SSD / HDD : Optimize for Windows Reinstallation
Then do a Custom install, with drive options to partition as you wish and format, leave Product Key blank during install, then do one of the workarounds given in the tutorial Bill2 links above after install to activate with Upgrade version key. Easiest is the registry edit - set a restore point beforehand.