windows authentication/local system logon - Oracle XA

hi,
I am fairly new, and a little confused.
If we want to allow applications such as oracle-xa to log on via dtcc etc, and the user login is currently set to use the local system account, does that user have to have an entry in ora_dba group to be allowed to connect to the database?
Can you not use windows authentication without the user being in that group.
I thought that group allowed any specified user to basically connect as SYS and with DBA priviledges. We don't really want the user to have dba priviledges, but do want them to be able to log on to select things from the database.
We are getting errors because the XA transaction manager is attempting to perform recovery with XA resource manager unsuccessfully. It cannot log on to get the in-doubt transactions I believe. But everything else that needs to be in place is in place, just that the windows user is not in the ora_dba group, but he is set to use the local system account to log in.