To me it sounds like you enabled encryption directly on the Windows folder. (Properties, Advanced attributes, encrypt contents).
On a stand-alone client, this feature uses a self-signed EFS certificate. To open or decrypt the files, you first need to migrate that certificate (including the private key) to your certificate store in Windows 8.
For migration, I found an old article that still applies.
http://technet.microsoft.com/en-us/library/cc722147(v=ws.10).aspx
If you lost the private key, I'm afraid that's bad news.