ORA-12560: TNS: protocol adapter error

C:\>sqlplus system/manager@xe

SQL*Plus: Release 11.2.0.2.0 Production

ERROR:

ORA-12560: TNS:protocol adapter error

Enter user-name:

C:\>set ORACLE_SID=xe

C:\>sqlplus system/manager@xe

Connected to:

Oracle Database 11g Express Edition Release 11.2.0.2.0 - Production

SQL> startup