connect system/password@xe doesn't work

Hi, I am not a DBA, I am interested in Oracle (4 weeks experience mainly at work), and am moving from Windows/SQL Server to Linux/Oracle for my 2 computer home network for learning.

I have a problem with my new Oracle XE installation. During installation I accepted all the defaults. My problem is that I can connect as some users from computer2 to computer1 (which has Oracle on it) but not all, specifically system/manager. I need to connect as system/manager onto computer1 from computer2 as computer1 doesn't have a monitor/vga card/keyboard. Do I need to set something somewhere (init.ora ??) or has my installation failed somehow. You will see what I have tried below for computer1 and computer2 and the Oracle error message I am getting. What I see tells me that the passwords and tnsnames.ora are setup properly too.

Thank you
John

computer1 (Oracle XE installed. Suse 9 x86):

 sqlplus system/manager_at_xe CONNECTS OK

 sqlplus hr/hr_at_xe CONNECTS OK

computer2 (Oracle client installed, Ubuntu x86):

 sqlplus hr/hr_at_xe CONNECTS OK

 sqlplus system/manager_at_xe GIVES THE FOLLOWING ERROR

john_at_ub04:~$ sqlplus system/manager_at_xe

SQL*Plus: Release 10.2.0.1.0 - Production on Sun Mar 11 10:35:19 2007

Copyright (c) 1982, 2005, Oracle. All rights reserved.

ERROR:
ORA-01017: invalid username/password; logon denied

Enter user-name: Received on Sun Mar 11 2007 - 05:46:39 CDT