No less than 7 security holes have been identified in JRE 1.5 prior to update 6, making older versions of the JRE completely out of the question from here on in. Also, the old versions MUST be completely uninstalled before installing update 6. Lastly, the full manual install is recommended over the so-called "web" install due to potential vulnerabilities (esp on NT-based systems).

Details available at the Sun site.