YES... YES !!!!!!!

Y O U--A R E--R I G H T !!!!

IT WORKED !

i tryied to drop a tablespace called "TOOLS" - no broblem, just wrote :
DROP TABLESPACE TOOLS INCLUDING CONTENTS AND DATAFILES;

worked just fine. then i tryied the same for the tablespace XDB and it didnt worked. so, i started to think that the XDB tbs is somehow more problematic to mannage (like SYSTEM tbs or somthing).

BUT Then You Came along and you were absolutly RIGHT !
I Dropped the user XDB and then the tbs :

DROP USER XDB CASCADE;
(Cascade - Drops all the user Objects as well.)
and then :

DROP TABLESPACE XDB INCLUDING CONTENTS AND DATAFILES;

Worked Just Fine !!!

Thank You guru_heaven You are ACE .