hi,
Applets are untrusted by browser because applets can access the hard disk of the running user. If proper permission are given they can create files/modify any file on your hard disk.. Now you can trust a signed applet i.e, digitally signed applets from trusted companies.
the major reason for making in untrusted is the ability to access hard disk.
hope this clears your doubt
-- Abdel Olakara
http://olakara.googlepages.com