Problem: When using your web browser to administer a JetDirect unit, the Java applets seem to load but you get java.security.AccessControlException errors and you can't navigate the menus.
Resolution: Uninstall Java from your machine and downgrade to Java 6 HERE. (The x86 version is what you want since most browsers are 32bit.) Remove the Java Update Scheduler from your startup (in the registry) so it doesn't upgrade you again. I recommend Autoruns for this.
Alternate Resolution: Use HP Web JetAdmin to administer the JetDirect box. It's sort of unwieldy in my opinion but it is nice to have in an environment where you have lots of HP printer devices on the network.