you need to delete the portal site and then do the restore. When a portal
exists in a virtual directory, you cannot perform a restore  activity using
SPSBackup.exe (same as the one u are using) utility. This is because the
database of the portal already exists. the restore activity creates the
database.

the restore will bring back all the data you had at the time of backup, cos it
is taking a database backup  and restoring the database.