There can be more than one Oracle instance on a single machine. In order to be able to distinguish these instances, Oracle uses a SID (System Identifier) which is a string. The SID can be set through the ORACLE_SID environment variable.
D:oracleproduct10.1.0>set ORACLE_SID=ORA10
On Windows, each instance requires a Windows service. This service must first be created with oradim: D:oracleproduct10.1.0Db_1>oradim -new -sid %ORACLE_SID% -intpwd MYSECRETPASSWORD -startmode M
D:oracleproduct10.1.0>set ORACLE_SID=ORA10
On Windows, each instance requires a Windows service. This service must first be created with oradim: D:oracleproduct10.1.0Db_1>oradim -new -sid %ORACLE_SID% -intpwd MYSECRETPASSWORD -startmode M