In Netscape the user doesn't even get the keys to the cage so you can't let the applets out into your system even if you wanted to. With the Applet Viewer it is possible to give your applets more access to the network. However they still can't do everything a C program can do.
Specifically a Java applet cannot write anything to a hard drive. It cannot write directly to memory, and it cannot introduce a virus into your system. A Java applet can use a lot of your CPU time though not 100% of it. It cannot crash your system (though a Java enabled browser can).
Posted in: Java