Oracle XE 10g problem

I used to use Oracle 10g enterprise version as my database, but I found it takes too much memory, so I switched to XE, it's working great until tonight.

When I start my weblogic server, I keep getting such error:

ORA-12519: TNS:no appropriate servicehandler found.

I have absolutely no idea what's this about, so google it, and found this: link

The fix is easy, and it works:

alter system set processes=150 scope=spfile;

Then restart your database.

If you run into an error like this: "ORA-01653: unable to extend table HAIBO_COMR.FORM by 8 in tablespace SYSTEM"

alter database datafile 'C:\ORACLEXE\ORADATA\XE\SYSTEM.DBF' RESIZE 1000M;
Make sure the highlighted part is the one in your system.

Other useful queries:

SELECT * FROM dba_data_files
purge recyclebin;
SELECT  tablespace_name,  SUM(bytes/1024/1024) FROM  dba_segments GROUP BY  tablespace_name;
select tablespace_name, sum(bytes)/1024/1024 from dba_free_space group by tablespace_name;