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.

Is there any other way of taking backups so that I can restore  without having to drop the portal site.
Is there a way to selectively restore say an events list or a document library

In case I have to delete the portal site... what is involved

if u want to backup  and restore  a single wss site u can use stsadm.exe. but you
cant selectively restore some list or document library. it is either a portal
or a site.

if you drop the portal and restore it with the backed up image. then you would
loose all changes you made after the backup.

other than that you get back everything. but i would advIse that you restart the
system before you do the restore because, it clears up the memory and that
helps the SPSBackup function better. This is an undocumented Microsoft feature.
;-))

I am attaching a document i prepared to help a frIend of mine go through the
backup restore activity. it is very basic, hope this helps you.