You can also always trying re-installing the latest version of Java at:

<A target="_blank" HREF=http://java.sun.com/webapps/download/AutoDL?BundleId=9504>http://java.sun.com/webapps/download/AutoDL?BundleId=9504</A>

Note that if you're seeing any type of message from the chat server (such as a login screen), your Java is probably working fine. If you never see any message from the chat server in that little center box, then your Java is not working fine and this may fix it.

Note this download is for Windows 98 computers or newer only.