In this post I like show the way to connect as sysdba without password for SYS user on Windows. The unique you need follow the next steps;
i) Make sure the Windows user is part of the group ORA_DBA.
ii) Into the SQLNET.ORA file, make sure the next line exist or insert it;
SQLNET.AUTHENTICATION_SERVICES = (NTS)
If any of the conditions above is not true You get a ORA-1031 error.
HTH – Antonio NAVARRO