It’s quite possible that their java virtual machine is corrupted. This year has been a bad one for security on a windows box and many exploits stem from IE and it’s integration with the operating system. If I was them I would try a new java plugin from http://www.java.com and see if that helps. If not I would download a better browser. Mozilla or Opera are both great. Prefer Mozilla’s Firebird myself. Then have them try entering the chat room. Also, you’ll want to remind them that they need to accept/grant the applet permission to run when the security window pops up showing the certificate for the applet.

Hope this helped some.