If you have the following message:
ORA-01552: cannot use system rollback segment for non-system tablespace 'USERS'
Then the undo tablespace is possibly corrupted.
Try the following:
create undo tablespace UNDOTBS2 datafile 'c:\app\vag\oradata\ora11g\UNDOTBS02.DBF' size 100000;
alter system set UNDO_TABLESPACE=UNDOTBS2;
drop tablespace UNDOTBS1 including contents and datafiles;
alter database datafile 'c:\app\vag\oradata\ora11g\UNDOTBS02.DBF' autoextend on maxsize unlimited;
Useful queries:
select * from sys.dba_tablespaces
select * from sys.dba_data_files