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Thats like mixing peanut butter and chocolate.
Someone jump in and correct me if I'm wrong.
You can view your db with cpanel, phpmyadmin whatever your host provides. I think you XOOPS db will still be there since you said you didn't delete it. I also doubt you find any postnuke tables installed because you stopped before that point. If any thing from postnuke did get installed you can just drop the tables.
If you mainfile.php is intact (chmod444) it didn't get overwritten so your probably ok. You might be able to remove the postnuke folders, and re-install the XOOPS update.
I'm being a little vague here because this is new to me too. If I can help you I will, but you might need someone more experienced than me.
don (el paso)
motto - green chili for everything!