If user run a signed applet, he/she will be prompt a security warning dialog box and need to give action to it, if user click on "Yes", the applet will have AllPermission to run on user's machine, which include write/delete file from local driver.

In window Vista OS, the process running in IE has low integrity level, which mean has less privilege than Administrator, the signed applet won't be able to writie/delete file from certern medium/high integrity level directory.

The signed applet in window Vista will have less privileges than those run in other window OS.