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If you would like to use the main index file to do this and you do not want it to include XOOPS functionality, you could create an index.php file which shows all this and "remove" the original index.php file (just keep it as a backup in case you change your mind).
Since the standard "Home" link in the menu block points to this main index site, I would suggest you to edit the template in which this is defined, and change it to your desired default XOOPS module. Otherwise a user would be directed to the intro page each time it clicks on the "Home" link.
This is one possibility, there are more of course.
I hope this was helpfull.
Martijn