Remember I only have the clues so far.
Some of the newer antivirus suites do block root directory modifications. There is rarely a reason to do this to C:\ so we'll have to boot safe mode or keep digging as to why this PC differs.
https://www.google.com/search?q=C%3A\+is+not+accessible+access+is+denied+Location+is+not+available finds many prior discussions and I rarely duplicate these prior solutions.
Right or wrong, we get to use research on this. We've covered the basics here and good to read you now have a spare admin account.
-> Remember that many commands such as the CMD prompt will do this unless we "run as Admin". This is nothing new but I find folk that need to talk about Admin and God issues at times. In Linux for example, root is almost God like but you may still need to chmod this or that to dust off an item.