After installing the 10g instant client if there is an protocol adapter error appear even when the tnsnames.ora is properly configured it could be that the tns_admin is not properly register.
You can simply add "String Value" in registry and it should solve your problem. Perform this and see if it works.
In the registry goto
HKEY_LOCAL_MACHINE->Software->Oracle->RightClick and add String Value and Name it TNS_ADMIN and give it the value of your correct Network Files.
For example if your TNSNAMES.ORA and SQLNET.ORA files are located under C:\oracle\product\10.2.0\client_1\network\admin then you should specify this value for TNS_ADMIN.