after proper installtion install oracle 8i (8.1.6.0.0) ON WIN2K

when i connect to oracle through sql
it gives message ora-12638 credential retrieval failed
after setting in sql net
sqlnet.authentication_services=(nts)
change to
sqlnet.authentication_services=(none)

after it i succesfully connect to oracle
but when i give
sys/*** as sysdba
on sql
it givew error ora-01031 insufficent privileges
i am connect with administrator user
and it is a member of ora_dba group
so whats the reason
but i connect internal succesfully and
done all work like(startup,shutdone)
but problem in connect sys/*** as sysdba on sql