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The way i did this was:
If you put your XOOPS site in to a sub folder eg XOOPS so it would be mysite/xoops. So if you put the page you want to be your first page in to the top level directory and rename it to index.htm so the server picks it up as the start page. Then in your main XOOPS site if you go to Mainfile.php and where you have your XOOPS root url change this so it includes the /xoops sub directory and now XOOPS will refresh to the origanal start page but you will see your new page when you first go to the site
If you want to get your new page to automaticaly refresh to the XOOPS site just add this little bit of code in to the html
<META HTTP-EQUIV="Refresh" CONTENT="0; URL=http://www.mysite.com/xoops">
CONTENT="0 number of seconds before refresh