Oracle Database Express Edition (XE)

Hello,
I recently installed Oracle DB XE on my PC running Windows XP Home Edition SP2.
It worked fine, till I tried to run the backup script.
I got the following error message:

Backup of database failed
Unknown log mode SP2-0640
Log file is at SP2-0640

Looking at similar messages into the forum, I understood it's a privilege problem; my OS user account (the one who installed Oracle) should be enlisted in the ORA_DBA group in order to be able to issue a 'connect / as sysdba' command.
Unfortunately, after some investigation I discovered that the 'Local Users and Groups' utility is not available in the Home Edition (available only in XP Pro :-( thanks to Microsoft).
So, as a workaround, I edited the Backup script, replacing every 'connect /' by 'connect sys/mypsw'.
The result is a little better, but not satisfactory. Hereafter is a copy of the logfile :

Recovery Manager: Release 10.2.0.1.0 - Production on Lun. Juil. 24 15:07:52 2006
Copyright (c) 1982, 2005, Oracle. All rights reserved.
Windows XP Version V5.1 Service Pack 2
CPU : 2 - type 586
Process Affinity : 0x00000000
Memory (Avail/Total): Ph:559M/1022M, Ph+PgF:1834M/2459M, VA:2003M/2047M
Starting with debugging turned off
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-00554: échec de l'initialisation du gestionnaire de récupération interne
RMAN-04005: erreur de la base de données cible :
ORA-01031: insufficient privileges

(As you can see, I'm a french speaking user).
Well, I feel Rman is also trying to use the ORA_DBA group feature.
So, is there a way to bypass that problem ?
Thanks in advance for your help,
André