I believe Microsoft's Java stopped somewhere around JDK 1.1.4, but the Rules Roundup
applet should actually work with Java 1.1. I just had a look and the class files are actually version 45, which is 1.1 compatible. I don't know if the applet works with Microsoft's JVM, though, and I suppose it's possible you have an older version of that as well.
But if you installed the latest Sun^H^H^HOracle JVM for Firefox, then that would almost certainly work.