Errors in file C:\ORACLEXE\APP\ORACLE\diag\rdbms\xe\xe\trace\xe_ora_832.trc:It seems that your installation is not succeeded.
ORA-01565: error in identifying file 'C:\oraclexe\app\oracle\oradata\XE\system.dbf'
ORA-27041: unable to open file
OSD-04002: unable to open file
O/S-Error: (OS 2) The system cannot find the file specified.
ORA-1503 signalled during: Create controlfile reuse set database "XE"
SQL> set ORACLE_SID=XEThat's not a Sql command, but an OS one, you have to execute it at OS prompt before starting Sql*Plus.