What comes back when you TNSPING the instance you are trying to connect to? I would think that from the limited info you have provided that there isn't an entry for your database instance in your TNSNAMES.ORA file.
There is an entry for database instance in tnsnames.ora, because I copied this file from original location to the new one.
Here is what I did:
1.downloaded zip files and unzipped them
2.removed all entries for PATH variable related to oracle and added new one pointed to the new directory with unzipped files
3.created new subdir c:\instantclient\network\admin
4.copied tnsnames and sqlnet to this folder(admin)from original location(c:\oracle\network\admin)