|I am assuming you used NTFS encryption.|
I think your files are gone.
Files that are encrypted are accessible only by the same account that encrypted them. Since you "changed Windows" that account no longer exists. An account with the same name and password on a different Windows installation is by means the same account.
This isn't a permissions issue so taking ownership of the files will not help.
Windows does make provision for accessing encrypted files after an OS upgrade. But this means backing up your encryption key before the upgrade. I am assuming you did not do this.