Try reinstalling Java. Remove the previous versions & then reinstall using the "Offline" installer which is about 18 MB to downld. You could also try the Java 1.6 beta
http://java.sun.com/javase/6/ (choose the JRE) & the offline installer is about 19 MB. Repeat the deleting preferences before opening FW. If still problems, try changing the FW compatibility to win 2000 or NT.