Windows 8 is a bitch...
You have two options:
a) reboot into safemode or
B) use an administrative account.
Option a) is pretty straight forward: open Run (windows key + R) and type 'msconfig'. Under the boot tab tick 'Safe boot' and reboot your PC. To exit safemode, you must follow the same procedure, except you untick 'Safe boot'.
Option B) is a little more involved, but it is something I found essential. I personally did this
because fuck security, I know what I'm doing. There are other ways, such as enabling the hidden administrator account
Once you've done one of the two options, you can change the permissions on the selected drive/folder/file. I assume you already know how to do this. Just remove the old user (it will be the one saying 'Account Unkown(blah)') and add the new one.