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More than likely the problem you're seeing stems from the fact that your web host has probably upgraded your version of PHP (probably several times
). If the site is as old as you say then the code you're using probably uses $HTTP_GET_VARS which is no longer supported in PHP and would need to be replaced by $_GET (the same with several other variables - $_POST, $_SERVER, etc).
There's several other changes in 'modern' installations of PHP (5.x series) that have changed the way PHP behaves and quite a few functions have been deprecated over the years (like all of the ereg functions).
There are also a lot of security improvements in both PHP and XOOPS (both core and modules) that need to be looked at too...
We'll need to know a little more about your installation before we can help guide you through an upgrade. Things like XOOPS version, any modules installed (and their versions), XOOPS theme used, PHP version, and MySQL version. Typically you can find an upgrade path to a more recent version of XOOPS and modules that will allow you to upgrade without having to recreate your site from scratch.