ORA-27100: shared memory realm already exists

This problem can come if you changed the sga_max_size parameter and system memory is not enough according to your settings.

Solution 1:
According to our favorite DBA Mr Amer Ali, apply Microsoft solution..which is just reboot your database server.
Hope It will work..

Solution 2 :
Open the command prompt then write the following command

c:> oradim -delete -sid orcl
then
c:\> oradim -new -sid orcl

This will remake the same sid of your.
Now try using that SID and then try to logging in.
Hope it will work......

Solution 3 :

This problem happened also after setting the sga_max mem too high.
Connect as sysdba to sqlplus
c:\>conn sys/sys@orcl as sysdba [ your dba username and password]
Connected to an idle instance.

now create pfile from spfile
sql>create pfile='c:\pfile.ora' from spfile;

Now edit your sga_max_size parameter in the c:\pfile.ora to a smaller value.

again create spfile from the pfile

sql>create spfile from pfile='c:\pfile.ora';

Now try to startup database
sql>startup
Hope it will work......