Getting ORA-01031:insufficient privileges during first time login to oracle 11.2.0.1.0 database on my windows 8 PC

I installed oracle database 11.2.0.1 on my windows 8.1 PC. I downloaded the zip files from oracle website viz. win64_11gR2_database_1of2 and win64_11gR2_database_2of2.

I followed the installation steps and while attempting to login through sysdba, i got the insufficient privileges issue. I used below command:

sqlplus "/as sysdba"

Please help me to resolve this issue.