Java APIs

hi i'm running widows xp pro, and i cant get into the chat rooms that i use any more,i used to be able to get in, but now when i go to justchat.co.uk/forum4, i cannot access the applet where you sign in your chat room name,i have looked in internet options in the temporary files, i click settings, then click view objects, it tells me that java runtime enviroment 1.3.1_01 is damaged, and i get another java runtime enviroment is dameged,i did uninstall both of these damaged javas, an downloaded a new javas it stated they where installed, but when i went to the chat room i got the following message, jave.lang.ClassNotFoundException.Forum.Class. loading java applet failed, i real am at a loss, i know nothing about java or how to re-install internet explorer, if any one can help with this problem it will be much appreciated, thanks.