New
#11
I hate to say this BUT! you may have to choose between the upgrade to repair type install or simply call it a day and slap a "Reset PC" on the system there! That option will be in the Advanced Recovery options while booted live from the 10 media if unable to get into the F8 boot options.
I had some fun trying to build a new BCD store on the second system when knocking out the original 7 Pro primary to move the clean installed 10's second forward and no results at making the drive bootable again until the second clean install got it all together! That's the sure fire way to get back into 10 without spinning in circles as well as having the need to put everything back on fresh from start again.
That would be the "Windows.old" folder some blog writers say can be renamed to "Windows.personal" or "Windows.backup" to secure personal files in a folder that won't be as easy to dump! The problem is that even after the renaming of that the Disk Cleanup or some 3rd party ware like the "crap cleaner" might end up nuking it anyways! "all my files are gone!"