ORA-27100

ORA-27100: Shared memory realm already exists

Cause: Tried to start duplicate instances, or tried to restart an instance that had not been properly shutdown

Action: Use a different instance name, or cleanup the failed instance's SGA

To resolve ORA-27100, you should execute the following instructions:
-Move the spfile from its default location or rename the spfile
-Restart the service
-Now the service will start the database using pfile, with old SGA settings. This would start your database successfully.
-Create the new spfile from pfile.

Another cause for this error is trying to start the database using a wrong user on windows platform. This happened to me when I tried to start the database using a domain user on windows 2000. After changing the user to LocalSystem, database was started successfully.