On Tuesday I blogged about htaccess and a custom 404 page. Also in that article, I hinted that your CMS should do your 404 page for you.

In this article I will explain how to overwrite your 404 page in Joomla 1.5, without changing core code.

As Joomla 1.5 is no longer supported this has been moved to the archive section.

404 page functionality is build into Joomla 1.5 making editing the 404 page easy. All you need to do is copy the 404 file located in your

/templates/system/
folder to the root of your current (default) template folder.

You also have the option of copying over their CSS, but I prefer to write mine from scratch.

Reference