after proper installtion install oracle 8i ( ON WIN2K

when i connect to oracle through sql
it gives message ora-12638 credential retrieval failed
after setting in sql net
change to

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