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chmod your mainfile.php to 777
Open your mainfile.php for starters and be sure that all paths, directories, database names and passwords are correct, edit where neccessary, although I doubt it has been touched.
Once you get that fixed, chmod it back to 444, and try to access your site.
What we need to do is get your XOOPS package able to access what it needs first, then we'll go the next step.