|Sorry, but I think you are out of luck.|
The files were encrypted with a certificate belonging to an account that no longer exists. Only that account can decrypt them. As far as the system is concerned those files do not belong to you and you are attempting unauthorized access. Even creating an account with the same name and password will not work.
Unless you have a backup of those files or you previously created a backup of the encryption certificate I don't see any way of accessing those files. The files were encrypted using the old, now non-existent certificate. Without that certificate Windows has no way to decrypt the files.