After you do "Shutdown abort" and before you do "startup mount" issue the "connect sys/password as sysdba" one more time. There is a possibility you are losing the privilege after the Abort.

To see if you are a member of ORA_DBA group on NT go to --

Start - Programs - Administrative Tools - User Manager for Domains

Find your username and see what Groups are assigned to him.
The GUI is very straight forward but if you don't have much experience you may need help from your NT admin.

- Rajeev