ERROR: ORA- 12638 Credential retrieval failed... ( Oracle Error)

Pls check the file, D:\Oracle\product\10.1.0\Client_1\network\ADMIN\sqlnet.ora and edit the following..

Original Entry - SQLNET.AUTHENTICATION_SERVICES= (NTS)

Modified Entry - SQLNET.AUTHENTICATION_SERVICES= (NONE)

Save the file and enjoy....