Java Plugin has an extension mechanism that will enable applets to install system-wide extensions into the jre/lib/ext (e.g c:\Program Files\java\jre1.6.0\lib\ext) directory. That won't work for window Vista OS, as protected mode can't write there.
Actually, plugin itself dosn't write to the lib/ext directory. The problem is that plugin is not allowed (unless running as administraitor) to call Runtime.exec() to launch the installer (which itself may write to lib/ext or wherever it wants).
For now we need to update the release notes to clairify this, and show a more meaningfull error dialog that informs the user why the installer cannot be run.