The response I got for Vista users is a suspicion you are either using an outdated version of Java, or something is hanging around in your Java cache that might be from an earlier version.
My first recommendation is to uninstall your existing version of Java from Add/Remove programs. Reboot. Visit:
http://www.java.com/en/download/manual.jsp and select the online version of Java for Windows Vista (should be the first or second thing in the list). Install.
Now try and see if you can enter. If it fails again, if there are any error codes or other information on screen, jot it down and post it here.