JRE build tried : Mustang b78
Vista build tried: beta2 build 5308, 5342

Steps to reproduce:

1 - Install mustang on a vista machine with Mozilla 1.7 browser installed.
2 - Open the Java Control Panel
3 - Go to the "Advanced" tab and click on "defaut java for browsers"
4 - Mozilla is unchecked by default.
5 - Now check Mozilla and click on Apply -> OK. Browser settings changed window comes up.
6 - Repeat steps 2 and 3

Actual Behavior - Mozilla is still unchecked
Expected Behavior - Mozilla should be checked.