Microsoft Internet Explorer (IE).
Make sure Java is enabled in all browsers. For IE, use Java 2 vXXX.
For Windows XP and Linux, download the runtime Edition (JRE) 1.4.2_xx for Windows XP and Linux from Sun: http://java.sun.com/j2se/1.4.2/download.html.
For Apple OS X (10.3), download from Apple: http://www.apple.com/support/downloads/javaupdate142.html.